staging: brcm80211: make use of linux error codes in srom.c

The source file srom.c used meaningless non-zero numbers to return
upon error conditions. These have been replaced by linux error codes.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>