bcma: use fallback sprom if sprom on card was not valid

Sometimes the PCIe card indicates that it has a sprom somewhere and we
are able to read the memory region, but it is empty and not valid. In
these cases we should try to use the fallback sprom as a last chance.

This is the case for the PCIe cards in my ASUS RT-N66U (BCM4706 + 2
times BCM4331) and I have heard of someone having the same problem with
an other PCIe card connected to an other Broadcom SoC.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>