Input: alps - fix compatibility with -funsigned-char

The AlpsPS/2 code previously relied on the assumption that `char` is a
signed type, which was true on x86 platforms (the only place where this
driver is used) before kernel 6.2. However, on 6.2 and later, this
assumption is broken due to the introduction of -funsigned-char as a new
global compiler flag.

Fix this by explicitly specifying the signedness of `char` when sign
extending the values received from the device.

Fixes: f3f33c67 ("Input: alps - Rushmore and v7 resolution support")
Signed-off-by: msizanoen <msizanoen@qtmlabs.xyz>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230320045228.182259-1-msizanoen@qtmlabs.xyz


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>