brcm80211: smac: inform mac80211 of the X2 regulatory domain

brcmsmac implements enforcement of regulatory constraints internally,
using a Broadcom-specific world roaming domain named X2. Besides being
duplication of functionality this can also conflict with mac80211's
regulatory implementation, as mac80211 is unaware of the X2 domain and
thus might apply a more restrictive domain.

This patch is the first step in making brcmsmac cooperate with
mac80211's regulatory support. X2 is registered as a custom domain with
mac80211, so that at least both implementations will be enforcing the
same set of constraints. The internal enforcement of rules is kept for
now; this will be converted over to relying on mac80211 regulatory
enforcement in later patches.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>