ath11k: set the BA buffer size to 256 when HE is enabled

This patch sets the correct BA buffer size when we are in HE mode.
Without this change we are not able to receive 256 bitmapped BA frames.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Pradeep Kumar Chitrapu <pradeepc@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>