usb: gadget: u_serial: Allow calling gserial_setup after init

Remove geserial_setup from the init section. The android gadget
driver calls it after probe, after userspace has configured the
gadget driver.

Signed-off-by: Benoit Goby <benoit@android.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>