Input: ff-core - silence an underflow warning

My static checker complains that "value" comes from the user in
evdev_do_ioctl() and we check that it's not too large here but we don't
check that it's negative.  It's harmless because the ->set_gain() and
->set_autocenter() functions truncate it to a valid u16 value, but we
may as well fix it just to make the static checker happy.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>