iio: Fix IIO_EVENT_CODE_EXTRACT_DIR bit mask

The direction field is set on 7 bits, thus we need to AND it with 0111 111 mask
in order to retrieve it, that is 0x7F, not 0xCF as it is now.

Fixes: ade7ef7b

 (staging:iio: Differential channel handling)
Signed-off-by: Cristina Ciocan <cristina.ciocan@intel.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>