usb: host: xhci: use slightly better list helpers

Replace list_entry() with list_first_entry() and list_for_each() with
list_for_each_entry(). This makes the code slightly more readable.

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>