Input: serio_raw - return proper result when serio_raw_read fails

serio_raw_read now returns (sometimes partially) successful number of
bytes transferred to the caller, and only returns error code to the
caller on completely failed transfers.

Signed-off-by: Che-Liang Chiou <clchiou@chromium.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>