ath11k: Add peer max mpdu parameter in peer assoc command

Add peer max mpdu length configuration support in peer_assoc_he
parameters. Noticed low throughput for the STA which
supports HE, HT and not VHT in MU-MIMO case without this
configuration

Signed-off-by: Tamizh Chelvam <tamizhr@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1588611900-21185-2-git-send-email-tamizhr@codeaurora.org