rt2x00: fix a crash bug in the HT descriptor handling fix

Commit "rt2x00: fix HT TX descriptor settings regression"
assumes that the control parameter to rt2x00mac_tx is always non-NULL.
There is an internal call in rt2x00lib_bc_buffer_iter where NULL is
passed. Fix the resulting crash by adding an initialized dummy on-stack
ieee80211_tx_control struct.

Cc: stable@vger.kernel.org # 3.7+
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>