usb: gadget: u_serial: fix switch off blocked

When a device is switched off by software, gserial_cleanup will
be called, and switch off will be blocked in this function
because wake_up_interruptible() in gs_close() can not wake_up
the wait_event() in gserial_cleanup(), it should be changed to
wake_up() to match the wait_event().

Signed-off-by: Haipeng YU <haipeng.yu@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>