brcmsmac: fix alignment constraints

sturct d11txh contains a ieee80211_rts structure, which is required to
have at least two byte alignment, and this conflicts with the __packed
attribute:

drivers/net/wireless/broadcom/brcm80211/brcmsmac/d11.h:786:1: warning: alignment 1 of 'struct d11txh' is less than 2 [-Wpacked-not-aligned]

Mark d11txh 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/20210204162852.3219572-1-arnd@kernel.org