USB: gadget: u_bam: Check for port_usb for NULL before ep_unconfig

In gbam2bam_connect_work(), tether_bridge_init() fails, error handling
is being done. As part of error handling, driver is calling
msm_ep_unconfig() for USB endpoints configured. But there is a chance
that disconnect happens during this time and port_usb becomes NULL.
Hence add a check for port_usb under spinlock and if it is not NULL,
don't call msm_ep_unconfig() as disconnect handling already takes care
of it.
Also check whether endless request is NULL or not before queueing it to
USB controller.

Change-Id: Ib7a563c79b1d08581fac9143cb41650ed6c24afd
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>