usb: dwc3: Fix lock-up on ID change during system suspend/resume

To reproduce the lock up do the following
- connect otg host adapter and a USB device to the dual-role port
so that it is in host mode.
- suspend to mem.
- disconnect otg adapter.
- resume the system.

If we call dwc3_host_exit() before tasks are thawed
xhci_plat_remove() seems to lock up at the second usb_remove_hcd() call.

To work around this we queue the _dwc3_set_mode() work on
the system_freezable_wq.

Fixes: 41ce1456 ("usb: dwc3: core: make dwc3_set_mode() work properly")
Cc: <stable@vger.kernel.org> # v4.12+
Suggested-by: Manu Gautam <mgautam@codeaurora.org>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>