can: ucan: fix alignment constraints

struct ucan_message_in contains member with 4-byte alignment
but is itself marked as unaligned, which triggers a warning:

drivers/net/can/usb/ucan.c:249:1: warning: alignment 1 of 'struct ucan_message_in' is less than 4 [-Wpacked-not-aligned]

Mark the outer structure to have the same alignment as the inner
one.

Link: https://lore.kernel.org/r/20210204162625.3099392-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>