usb: gadget: u_bam: Fix potential NULL port->port_usb dereference

Fix additional places in u_bam in which port->port_usb may be
accessed without checking for NULL pointer first. These checks
should be done with port->port_lock held.

Change-Id: Ia15edd8e760e21c95ca310523b29e0ba073d54db
Signed-off-by: Jack Pham <jackp@codeaurora.org>