wifi: rtl8xxxu: Add __packed to struct rtl8723bu_c2h

This struct is used to access a sequence of bytes received from the
wifi chip. It must not have any padding bytes between the members.

This doesn't change anything on my system, possibly because currently
none of the members need more than byte alignment.

Fixes: b2b43b78 ("rtl8xxxu: Initial functionality to handle C2H events for 8723bu")
Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/1a270918-da22-ff5f-29fc-7855f740c5ba@gmail.com