ath10k: Fix encoding for protected management frames

Currently the protected management frames are
not appended with the MIC_LEN which results in
the protected management frames being encoded
incorrectly.

Add the extra space at the end of the protected
management frames to fix this encoding error for
the protected management frames.

Tested HW: WCN3990
Tested FW: WLAN.HL.3.1-00784-QCAHLSWMTPLZ-1

Fixes: 1807da49 ("ath10k: wmi: add management tx by reference support over wmi")
Signed-off-by: Rakesh Pillai <pillair@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>