bcma: use fallback sprom if no on chip sprom is available

bcma should check for a fallback sprom every time it can not find a
sprom on the card itself or a normal external sprom mapped into the
memory of the chip. When otp sprom support was introduced it tried to
read out the sprom from the wireless chip also if no otp sprom was
available. This caused a Data bus error in bcma_sprom_get() when
reading out the sprom for the SoC.

This fixes a regression introduced in commit:

commit 10d8493c
Author: Arend van Spriel <arend@broadcom.com>
Date:   Tue Mar 6 15:50:48 2012 +0100

    bcma: add support for on-chip OTP memory used for SPROM storage

This patch was tested on a Netgear WNDR3400 (Broadcom BCM4718 SoC).

Reported-by: Nick Bowler <nbowler@elliptictech.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>