usb: gadget: u_serial: Avoid double unlock of serial_port_lock

usb: gadget: u_serial: Avoid double unlock of serial_port_lock

Avoid unlocking serial_port_lock twice in gserial_suspend(), this can
occur if gserial_wakeup_host() fails. And since wakeup is performed
outside spinlock, check if the port is valid before proceeding again.

Fixes: 3baea29dc0a7 ("usb: gadget: u_serial: Implement remote wakeup capability")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/aBHatifO5bjR1yPt@stanley.mountain/
Signed-off-by: Prashanth K <prashanth.k@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250506104912.3750934-1-prashanth.k@oss.qualcomm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>