[media] dvb_usb_lmedm04: don't crash if firmware is not loaded

There is a missing error handling when no firmware file found.

It seems that this is more of a problem with udev-182+.

However, so far udev-182 is only a problem on first ever plug.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Tested-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>