USB: xhci: Performance - move xhci_work() into xhci_irq()

When we move xhci_work() into xhci_irq(), we don't need to read the operational
register status field twice.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>