brcmsmac: fix reading of PCI sprom contents

It appears that you can only read the sprom contents with aligned 16-bit
reads: anything else causes at least some versions of the broadcom
chipset to abort the PCI transaction, returning 0xff.

This apparently doesn't trigger very often, because most setups don't
use an external srom chip, and the OTP sprom loading doesn't have this
issue.  But at least the current 11" Macbook Air does trigger it, and
wireless communications were broken as a result.

Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>