usb: dwc3: Use high priority worker as bottom half handler

Latency involved in handling dwc3 interrupt in real time threaded
interrupt is sometimes resulting into RT throttling. Hence queue
bottom half work from dwc3 hard irq handler to a high priority
workqueue. Also, update the corresponding irq flag in host
controller platform driver.

Change-Id: If65a522efb6d792eea7fa7afc4c5e4dcd1cb6fb8
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Mayank Rana <mrana@codeaurora.org>