USB: Add a usb_pipe_endpoint() convenience function

Converting a pipe number to a struct usb_host_endpoint pointer is a little
messy.  Introduce a new convenience function to hide the mess.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
index 79d1cdf..d8329eb 100644
--- a/drivers/usb/core/message.c
+++ b/drivers/usb/core/message.c
@@ -226,8 +226,7 @@
 	struct urb *urb;
 	struct usb_host_endpoint *ep;
 
-	ep = (usb_pipein(pipe) ? usb_dev->ep_in : usb_dev->ep_out)
-			[usb_pipeendpoint(pipe)];
+	ep = usb_pipe_endpoint(usb_dev, pipe);
 	if (!ep || len < 0)
 		return -EINVAL;