bcma: return error in bcma_sprom_get() when fallback fails

When not SPROM is available a fallback mechanism is used. However,
when that fails the code currently continues. This patch assures
that the bcma_sprom_get() function aborts when that happens.

Cc: Rafal Milecki <zajec5@gmail.com>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>