const-qualify interp_instance structs in interpolation functions

[escherstair]

Interpolation functions take as an input an interp_instance struct.
As an example,

CMSIS-DSP/Source/InterpolationFunctions/arm_linear_interp_f32.c

Lines 79 to 81 in 99e15d3

	float32_t arm_linear_interp_f32(
	arm_linear_interp_instance_f32 * S,
	float32_t x)

As far as I understand, this interp_instance is not modified by the function, and so the pointer can be declared as pointer to const.

float32_t arm_linear_interp_f32(
  const arm_linear_interp_instance_f32 * S,
  float32_t x)

In this way the code is more expressive, and a const struct can be passed as an input without any warning about casting-away its const-ness.

[christophe0606]

Corrected in latest commit