usb: fix implicit usage of gfp.h in host/xhci-hub.c

To fix this build error on ARM:

drivers/usb/host/xhci-hub.c: In function 'xhci_stop_device':
drivers/usb/host/xhci-hub.c:261: error: 'GFP_NOIO' undeclared (first use in this function)
make[4]: *** [drivers/usb/host/xhci-hub.o] Error 1

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>