usb: host: xhci: remove unused trb var in xhci_irq()

The union xhci_trb *trb variable is defined and assigned
inside the xHCI IRQ handler function but is never used.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>