Input: focaltech - fix array out-of-bounds in focaltech_process_rel_packet

Input: focaltech - fix array out-of-bounds in focaltech_process_rel_packet

Make finger2 (and also finger1) unsigned, so that if the finger index in
the packet is 0 then subtracting 1 creates an array index which overflows
above the existing check for FOC_MAX_FINGERS, as the existing comment says
it should, instead of writing to state->fingers[-1].

Fixes: 05be1d079ec0 ("Input: psmouse - support for the FocalTech PS/2 protocol extensions")
Signed-off-by: Richard Davies <richard@arachsys.com>
Link: https://patch.msgid.link/20260701190932.14960-1-richard@arachsys.com
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>