usb: host: xhci: simplify irq handler return

Instead of having several return points, let's use a local variable and
a single place to return. This makes the code slightly easier to read.

[set ret = IRQ_HANDLED in default working case  -Mathias]
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>