brcm80211: smac: only provide valid regulatory hint

The driver provides a regulatory hint to cfg80211 as obtained from the
SPROM. Mostly, this will be a two-letter ISO country code. However, it
may obtain special country code similar to the world regulatory domain
as used in cfg80211. This patch avoids setting these special codes as
the hint is lost to cfg80211.

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>