cfg80211: remove macro ASSERT_RDEV_LOCK(rdev)

Macro ASSERT_RDEV_LOCK(rdev) is equal to ASSERT_RTNL(), so replace it
with ASSERT_RTNL() and remove it.

Signed-off-by: Zhao, Gang <gamerh2o@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>