brcm80211: smac: use mac80211 channel data for tx power limits

Currently the limits from the internal X2 domain are used, regardless
of what regulatory rules are in effect. Instead use the power limits set
by the higher-level regulatory support.

The rules for the MIMO power limits are still always derived from the
world domain, pending guidance from Broadcom as to how these need to be
handled. This will be fixed later, but using the limits from the world
domain works for now.

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>