platform: msm: usb_bam: Fix potential use-after-free in connect_pipe

In the connect_pipe() failure path, the allocated pipe is freed but
the pointer variable is not reset creating a dangling pointer and
potential UaF if it is later accessed.  Fix it by assigning it to NULL.

Change-Id: Iae9fb05ce819fc94839180762393fa18aaecdd60
Signed-off-by: Jack Pham <quic_jackp@quicinc.com>