HID: prodikeys: kfree() NULL pointer cleanup

No need for freeing pm in case when it's not allocated.

Reported-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Bojan Prtvar <prtvar.b@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>