inv_mpu: Makefile/Kconfig: Fix up & simplify some configs

Seems Samsung might have derp'd a few config-related changes.
This allows for proper usage of the MPU_SENSORS_MPU6050_GYRO config
and makes selftest parts optional.

Fixes:
  drivers/misc/inv_mpu/Kconfig:52:warning: defaults for choice values not supported

Change-Id: I80c476c3e8723403a580c9d25ecfe38bc84ddb3b
Signed-off-by: Luis Cruz <ljc2491@gmail.com>