m68k: sun3: Add missing asmlinkage to sun3_init()

sun3_init() is called from assembly code, so it should be marked
asmlinkage for documentation purposes.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/9a174020de2c55e108be9bd25fcdec3e48d655c4.1694613528.git.geert@linux-m68k.org