staging: emxx_udc: allow modular build

A change to the usb gadget core allowed certain API functions to be
part of a loadable module, which breaks having emxx_udc built-in:

drivers/staging/built-in.o: In function `nbu2ss_drv_probe':
(.text+0x2428): undefined reference to `usb_ep_set_maxpacket_limit'

The original patch already fixed tons of other cases that have the
added dependency but apparently missed this one that now appears
in an ARM allmodconfig build.

This patch makes the symbol "tristate", which lets the Kconfig
dependency tracking handle it correctly. To make the module
actually usable, I also revert 0af61e66 ("drivers/staging:
make emxx_udc.c explicitly non-modular"), which Paul Gortmaker
added after noticing that the Kconfig symbol was 'bool'.
Compared to the original version however, I leave out the
'__exit' annotation on the remove callback, as Paul pointed
out that this was incorrect.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 5a8d651a ("usb: gadget: move gadget API functions to udc-core")
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>