wireless: Save original maximum regulatory transmission power for the calucation of the local maximum transmit power

The local maximum transmit power is the maximum power a wireless device
allowed to transmit. If Power Constraint is presented, the local maximum
power equals to the maximum allowed power defined in regulatory domain
minus power constraint.

The maximum transmit power is maximum power a wireless device capable of
transmitting, and should be used in Power Capability element (7.3.2.16
IEEE802.11 2007).

The transmit power from a wireless device should not greater than the
local maximum transmit power.

The maximum transmit power was not calculated correctly in the current
Linux wireless/mac80211 when Power Constraint is presented.
Signed-off-by: Hong Wu <hong.wu@dspg.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>