usb: gadget: fixes for "use config_ep_by_speed() instead of ep_choose()"

Since f_serial is now checking if the endpoint descriptors are
null in the set_alt function, need to set them to null on
ep_disable in case of cable disconnect.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>