staging:iio:adc:ad7291: Fix AD7291_T_SENSE_MASK

AD7291_T_SENSE_MASK must set bit-7.
Change definition style.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>