Input: ads7846 - correct the value got from SPI

According to the touch controller spec, SPI return a 16 bit value, only 12
bits are valid, they are bit[14-3].

The value of MISO and MOSI can be configured when SPI is in idle mode.
Currently this touch driver assumes the SPI bus sets the MOSI and MISO in
low level when SPI bus is in idle mode. So the bit[15] of the value got
from SPI bus is always 0. But when SPI bus congfigures the MOSI and MISO in
high level during the SPI idle mode, the bit[15] of the value get from SPI
is always 1. If bit[15] is not masked, we may get the wrong value.

Mask the invalid bit to make sure the correct value gets returned.
Regardless of the SPI bus idle configuration.

Signed-off-by: Andrey Gelman <andrey.gelman@compulab.co.il>
Signed-off-by: Haibo Chen <haibo.chen@freescale.com>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>