icm42688: Fix divide by zero potential

There were code paths that could have lead to divide by zero given an
invalid scale setting for accel or gyro. In practice this should be an
invalid setup even before getting to these conversion functions. The
conversion functions now better show all valid values are accounted for
by using CODE_UNREACHABLE in the default case.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>