Staging: iio/accel: Changed data type of mask in store_measurement_mode() to u8

In sca3000_store_measurement_mode() we parse a value from a string
buffer via kstrtou8, and store the parsed value into a u8 after
and-ing it with mask.

As we are only interested in the lowest two bits here and mask is
initialized with a fixed value 0x03, mask may as well be a u8.

Signed-off-by: Andreas Ruprecht <rupran@einserver.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>