Input: atmel_mxt_ts - do not read extra (checksum) byte

atmel_mxt devices will send a checksum byte at the end of a message if
the MSB of the object address is set.
However, since this driver does not set this bit, the checksum byte
isn't actually sent, so don't even try to read it.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>