firmware: arm_scmi: Remove __exit annotation

firmware: arm_scmi: Remove __exit annotation

virtio_scmi_exit() is only called from __exit function, so the annotation
is correct, but when the driver is built-in, the section gets discarded
and the reference from a callback pointer causes a link-time error:

`virtio_scmi_exit' referenced in section `.rodata' of drivers/firmware/arm_scmi/virtio.o:
	defined in discarded section `.exit.text' of drivers/firmware/arm_scmi/virtio.o

I could not figure out a better workaround, so let's just remove that
annotation even if it wastes a couple of bytes in .text.

Link: https://lore.kernel.org/r/20210920100301.1466486-2-arnd@kernel.org
Fixes: 5c0279bd1b6b ("firmware: arm_scmi: Add virtio transport")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>