usb: u_data_bridge: Check for USB cable connect with gbridge_port_write()

There is NULL pointer crash seen due to accessing port->port_usb from
gbridge_port_write() when port bridge application's write thread is
pre-empted and USB cable disconnect is being performed. Fix this issue
by adding check for USB cable is connected or not and handling USB
request accordingly.

Change-Id: Ide14de298b7186cbe7b7af1e99c8c47bc389130b
Signed-off-by: Mayank Rana <mrana@codeaurora.org>