mac80211: remove user_power_level from driver API

I missed this during review of "mac80211: Fix tx power setting",
the user_power_level shouldn't be available to the driver but
rather be an internal value used to calculate the value for the
driver.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>