Input: silead_gsl1680 - use "silead/" prefix for firmware loading

The silead touch-controller ICs use a different firmware per digitizer /
tablet model. So there are going to be quite a few of then and they really
should be under a separate subdir.

This commit prefixes the default firmware names with "silead/" just like
we are already doing for devicetree specified firmware names.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>