brcm80211: smac: do not use US as fallback regulatory hint

The brcmsmac driver provides the country code from sprom as a regulatory
hint to cfg80211. When brcmsmac does not find a country code entry in
the sprom it passes 'US' as regulatory hint. Better approach is to rely
on the world regulatory domain in cfg80211/crda.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Brett Rudley <brudley@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>