usb: xhci: move xhci.h to include usb

The xhci.h header file is currently located under drivers/usb/xhci
Move it to the include/usb folder to make it available to drivers that
are not under drivers/usb/xhci

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index b2cfd94..119b418 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -19,7 +19,7 @@
 #include <asm/unaligned.h>
 #include <linux/errno.h>
 
-#include "xhci.h"
+#include <usb/xhci.h>
 
 /**
  * Is this TRB a link TRB or was the last TRB the last TRB in this event ring