Revert "xhci-plat: Update IRQ flag for primary HCD"

This reverts 'commit f05946d2 ("xhci-plat:
Update IRQ flag for primary HCD")'.

The original change depends on commit bb2195e5 ("usb:
dwc3: Change dwc3 irq to oneshot threaded irq") which
has been reverted due to USB tputs regression. Revert this
one to make the revertion completely or USB host can't work.

Change-Id: I2e506a479fb633d3ece338427664d4c75497519d
CRs-Fixed: 1112306
Signed-off-by: Leon Peng <liangyan@codeaurora.org>