wl3501: fix alignment constraints

struct wl3501_80211_tx_hdr contains a ieee80211_hdr structure, which is
required to have at least two byte alignment, and this conflicts with
the __packed attribute:

wireless/wl3501.h:553:1: warning: alignment 1 of 'struct wl3501_80211_tx_hdr' is less than 2 [-Wpacked-not-aligned]

Mark wl3501_80211_tx_hdr 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/20210204162653.3113749-1-arnd@kernel.org