USB: gadget driver unbind() is optional; section fixes; misc

Allow gadget drivers to omit the unbind() method.  When they're
statically linked, that's an appropriate memory saving tweak.

Similarly, provide consistent/simpler handling for a should-not-happen
error case:  removing a peripheral controller driver when a gadget
driver is still loaded.  Such code dates back to early versions of the
first implementation of the gadget API, and has never been triggered.

Includes relevant section annotation fixs for gmidi.c, file_storage.c,
and serial.c; we don't yet have an "init or exit" annotation.  Also
some whitespace fixes in gmidi.c (space at EOL, before tabs, etc).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>