usb: core: Enable xhci irq after starting controller

There is a possibility of port change event triggering
xhci irq as soon as halt bit is cleared in xhci_start().
As a result before xhci state is changed from XHCI_STATE_HALTED
to 0 port change event keeps on generated until port status is
acknowledged. This does not allow xhci_start() to finish and handle
the port change event if irq is keep on getting fired on same core
where xhci_start() is running. Fix this issue by disabling irq before
starting controller and enable it back after clearing halt bit.

Change-Id: I798620f99a7ba522258455642e6e8091ebf2cd34
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>