staging: iio: light: isl29028: fix correct mask value

The mask value in the read_raw/write_raw is absolute
value, not the bit position value.
Fixing this in the implemented function to check value,
not with the bit position value.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>