staging: brcm80211: improved checks on incompatible firmware

The return status of wl_ucode_init_buf() is now being used. Also a bug in
firmware validation, which could lead to incompatible firmware not being
rejected by the driver, has been fixed. Comment from Dan Carpenter on using
negative error values has been incorporated in this commit.

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