[driver] Explicitly cast enum class assignments

[salkinium]

Assigning an enum class value with another enum class value is not
type compatible, and must be explicitly cast before assignment.

This bug seems to have been ignored by GCC until the 6.1 release.
Clang (used for hosted unit tests) does not ignore this bug.

Thanks to @daniel-k for reporting this bug in #151.
cc @ekiwi @strongly-typed

[salkinium]

Another UX fail is the inablity to use the overloaded flags at assignment time:

Given this initial register (from the LIS3DSH driver here):

/// CTRL_REG4 default value is 0x07
enum class
Control4 : uint8_t
{
    ODR3 = Bit7,
    ODR2 = Bit6,
    ODR1 = Bit5,
    ODR0 = Bit4,
    ODR_Mask = Bit7 | Bit6 | Bit5 | Bit4,

    BDU = Bit3,     ///< Block data update.
    ZEN = Bit2,     ///< Z axis enable
    YEN = Bit1,     ///< Y axis enable
    XEN = Bit0,     ///< X axis enable
};
XPCC_FLAGS8(Control4);

You must then cast each flag individually to int before ORing, instead of being able to use the operator overloading:

enum class
MeasurementRate : uint8_t
{
    Off = 0,
    Hz3_125 = int(Control4::ODR0),
    Hz6_25 = int(Control4::ODR1),
    Hz12_5 = int(Control4::ODR1) | int(Control4::ODR0),
    Hz25 = int(Control4::ODR2),
    Hz50 = int(Control4::ODR2) | int(Control4::ODR0),
    Hz100 = int(Control4::ODR2) | int(Control4::ODR1),
    Hz400 = int(Control4::ODR2) | int(Control4::ODR1) | int(Control4::ODR0),
//  Obviously it's not possible to OR first, then cast!1!!
//  Hz400 = int(Control4::ODR2 | Control4::ODR1 | Control4::ODR0),
    Hz800 = int(Control4::ODR3),
    Hz1600 = int(Control4::ODR3) | int(Control4::ODR0),
};

On the other hand, semantically speaking, the above example is pointless.
The user is never supposed to use the FifoControl::FMx bit manually, that's what the FifoMode is for.
Therefore using the xpcc::Bitx directly in FifoMode seems like a better idea.
That would translate to this:

/// CTRL_REG4 default value is 0x07
enum class
Control4 : uint8_t
{
//  No ODR bits!
    BDU = Bit3,     ///< Block data update.
    ZEN = Bit2,     ///< Z axis enable
    YEN = Bit1,     ///< Y axis enable
    XEN = Bit0,     ///< X axis enable
};
XPCC_FLAGS8(Control4);

enum class
MeasurementRate : uint8_t
{
    Off = 0,
    Hz3_125 = Bit4,
    Hz6_25 = Bit5,
    Hz12_5 = Bit5 | Bit4,
    Hz25 = Bit6,
    Hz50 = Bit6 | Bit4,
    Hz100 = Bit6 | Bit5,
    Hz400 = Bit6 | Bit5 | Bit4,
    Hz800 = Bit7,
    Hz1600 = Bit7 | Bit4,
};
// For completeness
using MeasurementRate_t = Configuration<Control4_t, MeasurementRate, 0b11110000>;