USB: gadget: qdss: Fix null pointer dereference

Unless bam_to_bam_ipa transport is used ipa_data_wq
won't allocate. During unbind of qdss function checks
the completion of ipa_data_wq leads to NULL pointer
dereference. Fix this by add proper check.

CRs-Fixed: 1110001
Change-Id: Iaf11a2b7575fa31f984b572d6bfffa44a38d9f8d
Signed-off-by: Arumuga Durai A <cadurai@codeaurora.org>