xhci: Do not queue set tr dequeue command when slot is disabled

Set tr dequeue command is queued without checking  the slot state.
XHCI spec does not allow driver software to ring doorbell on a
disabled slot. Hence check for slot state before queuing the command
to controller

CRs-Fixed: 685588
Change-Id: Ie4980ba8b93e8753193b5cfb5e7a3045b1c04145
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>