iio:inv-mpu6050: Fix inconsistency for the scale channel

Fix inconsistency in the semantics of the scale attribute.
For scale the write_raw function was considering the scale table index
and writing the appropriate value into the range register, while
for read_raw it was outputting the actual scale.
Fix this behaviour and adhere to the iio ABI specification.

Signed-off-by: Adriana Reus <adriana.reus@intel.com>
Reviewed-by: Viorel Suman <viorel.suman@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>