ath11k: Fix fw assert by setting proper vht cap

After setting fixed vht-rate if new station is trying to
assoc with mu_bfee cap, or if a sta is already connected
with mu_bfee cap then set the fixed vht-rate and
reconnecting the sta, FW assert is happening.

So to avoid this, reset the MU_BEAMFORMEE bit in vht->caps,
if mcs_index is invalid for nss 1.

Signed-off-by: Ritesh Singh <ritesi@codeaurora.org>
Signed-off-by: Maharaja Kennadyrajan <mkenna@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1586538405-16226-2-git-send-email-mkenna@codeaurora.org