usb: gadget: u_bam: Fix incorrect nested spinlock

commit 555513e8 ("usb: gadget: u_bam: Remove support for
BAM-DMUX") removed the additional port_lock_ul/dl spinlocks and
consolidated their usage to the single port_lock wherever possible.
However, gbam_free_rx_buffers() is called with that lock already
held, so remove the extra lock acquisition to fix a recursion bug.

Change-Id: I29aefdb4f80c3b033bda56fbcdcacaa322d2763b
Signed-off-by: Jack Pham <jackp@codeaurora.org>