HID: ntrig: explain firmware quirk

Commit 6549981b ("HID: fix N-trig touch panel with recent firmware")
adds a quirk that is needed for devices with more recent firmware so that
they become operational.

As it's not directly obvious from the code why is it needed, a comment
is worthwile.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>