ath10k: leave MMIC generation to the HW

Apparently HW doesn't require us to generate MMIC
for TKIP suite.

Each frame was 8 bytes longer than it should be
and some APs would drop frames that exceed 1520
bytes of 802.11 payload. This could be observed
during throughput tests or fragmented IP traffic.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>