swim3: mark swim3_init() static

This is the module init function, which by definition is used only
locally, so mark it static to avoid a warning:

drivers/block/swim3.c:1280:5: error: no previous prototype for 'swim3_init' [-Werror=missing-prototypes]

Reviewed-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>