bcma: fix error code handling on 64 Bit systems

On most 64 Bit systems unsigned long is 64 bit long and then -MAX_ERRNO
is out of the range of a u32 used to store the error code in.
This patch casts the -MAX_ERRNO to a u32 instead.

This fixes a regression introduced in:
commit fd4edf197544bae1c77d84bad354aa7ce1d08ce1
Author: Hauke Mehrtens <hauke@hauke-m.de>
Date:   Mon Jul 15 13:15:08 2013 +0200

    bcma: fix handling of big addrl

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