Input: ads7846 - fix unaligned access on 7845

req->sample[1] is not naturally aligned at word boundary, and therefore we
should use get_unaligned_be16() when accessing it.

Fixes: 3eac5c7e ("Input: ads7846 - extend the driver for ads7845 controller support")
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>