USB: gadget: u_bam: Fix NULL pointer deference during disconnect

It might happen that disconnect and notify callback from BAM DMUX
driver race each other and result in crash due to NULL pointer
dereference in notify callback. Hence fix this by having check
before pointer dereference and free skb if pointer is NULL.

Change-Id: Id87fd49946405cd490cf2c18e4a9ddd5ca7ea0a0
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>