ath10k: align ath10k_htt_txbuf structures

With W=1 GCC warns:

drivers/net/wireless/ath/ath10k/htt.h:1746:1: warning: alignment 1 of 'struct ath10k_htt_txbuf_32' is less than 4 [-Wpacked-not-aligned]
drivers/net/wireless/ath/ath10k/htt.h:1753:1: warning: alignment 1 of 'struct ath10k_htt_txbuf_64' is less than 4 [-Wpacked-not-aligned]

Fix that by using __align(4). Compile tested only.

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>