[intelxl] Allow for arbitrary placement of ring tail registers

The virtual function transmit and receive ring tail register offsets
do not match those of the physical function.  Allow the tail register
offsets to be specified separately.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/intelxl.h

@@ -680,6 +680,8 @@ struct intelxl_ring {
 
 	/** Register block */
 	unsigned int reg;
+	/** Tail register */
+	unsigned int tail;
 	/** Length (in bytes) */
 	size_t len;
 	/** Program queue context