USB: xhci: fix some compiler warnings in xhci.h

This fixes the warning:
drivers/usb/host/xhci.h:1083: warning: passing argument 1 of ‘xhci_to_hcd’ discards qualifiers from pointer target type
drivers/usb/host/xhci.h:1083: warning: passing argument 1 of ‘xhci_to_hcd’ discards qualifiers from pointer target type

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>