usb: Refactor irq enabling out of usb_add_hcd()

Refactor out the code in usb_add_hcd() to request the IRQ line for the
HCD.  This will only need to be called once for the two xHCI roothubs, so
it's easier to refactor it into a function, rather than wrapping the long
if-else block into another if statement.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>