usb: gadget: fix g_serial regression

Commit "usb: gadget: use config_ep_by_speed() instead of
ep_choose()" broke g_serial in "non ACM nor OBEX"
mode. Apply a trivial fix on usb endpoints discovery.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>