aoc: usb: prevent a dereference in free_transfer_ring

The pointer 'ctrl_ctx' returned from call to function
'xhci_get_input_control_ctx' may be NULL and will be
dereferenced at the following using.

Bug: 206921381
Test: build pass and headset workable
Signed-off-by: Puma Hsu <pumahsu@google.com>
Change-Id: I56e6a4e08995ba3efd0dd98a616e01082ea92162