usb: host: Verify sg before accessing it in qh_urb_transaction

Fix code quality issues found by the automatic checking tool.
Verify safe access to sg in qh_urb_transaction.

Change-Id: I6ce40e8548449aebd628c72bc2b9cda1b2861ba9
CRs-Fixed: 566927
Signed-off-by: Danny Segal <dsegal@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>