staging:iio:spear_adc: Remove unused variable

Remove the scale_mv variable from the read_raw() callback. Fixes the following
warning:
	drivers/staging/iio/adc/spear_adc.c: In function 'spear_read_raw':
	drivers/staging/iio/adc/spear_adc.c:149:6: warning: unused variable 'scale_mv'

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>