ssb: mark ssb_bus_register as __maybe_unused

The SoC variant of the ssb code is now optional like the other
ones, which means we can build the framwork without any
front-end, but that results in a warning:

drivers/ssb/main.c:616:12: warning: 'ssb_bus_register' defined but not used [-Wunused-function]

This annotates the ssb_bus_register function as __maybe_unused to
shut up the warning. A configuration like this will not work on
any hardware of course, but we still want this to silently build
without warnings if the configuration is allowed in the first
place.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 845da6e5 ("ssb: add Kconfig entry for compiling SoC related code")
Acked-by: Michael Buesch <m@bues.ch>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>