mwl8k: fix alignment constraints

mwl8k: fix alignment constraints

sturct mwl8k_dma_data contains a ieee80211_hdr structure, which is required to
have at least two byte alignment, and this conflicts with the __packed
attribute:

vers/net/wireless/marvell/mwl8k.c:811:1: warning: alignment 1 of 'struct mwl8k_dma_data' is less than 2 [-Wpacked-not-aligned]

Mark mwl8k_dma_data itself as having two-byte alignment to ensure the
inner structure is properly aligned.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210204162813.3159319-1-arnd@kernel.org