Input: psmouse - fix OOB access in Elantech protocol

The kernel only allocate 5 MT slots; check that transmitted slot ID
falls within the acceptable range.

Link: https://lore.kernel.org/r/ZFnEL91nrT789dbG@google.com
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>