carl9170: Remove redundant protection check

The carl9170_op_ampdu_action() function is used only by the mac80211
framework.
Since the mac80211 already takes care of checks and properly serializing
calls to the driver's function there is no need for the driver to do the same
thing.

Signed-off-by: Andreea-Cristina Bernat <bernat.ada@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>