greybus: tag core init and exit functions

The top-level functions gb_init() and gb_exit() are tagged with
__init and __exit, respectively.  These functions call a few
other functions that are similarly used only at initialization
and termination time.  So mark those functions accordingly.

Note that, because gb_ap_exit() and gb_debugfs_cleanup()
are called by gb_init() in error paths, these functions
cannot be declared with the __exit attribute.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>