b43: only hardcode LED behavior if SPROM doesn't encode any

Only hardcode the LED behavior if the SROM doesn't provide any for all
LEDs of the card. This avoids instantiating LED triggers for unconnected
LEDs, while (hopefully) keeping things working for old cards with a
blank SROM.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>