adafruit_bus_device: SPI object doesn't have spi property (and adafruit_sdcard assumes it)

[jepler]

Reported by @Neradoc on Discord, this issue is separate from other problems encountered with adafruit_bus_device.

It wasn't immediately clear why adafruit_sdcard uses this property (efficiency?) but as the property was not _spi it was arguably fair game. Possible resolutions that occurred to me:

• change adafruit_sdcard to not use the property and leave in-core adafruit_bus_device unchanged for the purposes of this PR
• add the property to adafruit_bus_device and leave adafruit_sdcard unchanged
• change both even though it's redundant
  I didn't find any uses of adafruit_bus_device.i2c_device.I2C.i2c with my quick grepping in the Learn system code and Bundle, but the same concern may apply there.

[tannewt]

This no longer blocks 6.1.0 because we've disabled native bus device for now.

[dhalbert]

Fixed by #4114.