usb: xhci_msm: Unblock xhci bus supsend during xhci shutdown

xhci bus suspend depends on lpm irq (l2-in) and for some reason
its not triggered during system shutdown resulting in panic. Avoid
panic by unblocking suspend thread in shutdown callback.

CRs-fixed: 647621
Change-Id: I90a78e2a321d50050053cecfa66de80a9002a32f
Signed-off-by: Guang Jian Zeng <gzeng@codeaurora.org>