macintosh/ams: mark ams_init() static

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

drivers/macintosh/ams/ams-core.c:179:12: error: no previous prototype for 'ams_init' [-Werror=missing-prototypes]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230810141947.1236730-7-arnd@kernel.org