pasemi_mac: Fix TX ring wrap checking

The old logic didn't detect full (tx) ring cases properly, causing
overruns and general badness. Clean it up a bit and abstract out the
ring size checks, always making sure to leave 1 slot open.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
diff --git a/drivers/net/pasemi_mac.h b/drivers/net/pasemi_mac.h
index fbbc17a..c5b0adb 100644
--- a/drivers/net/pasemi_mac.h
+++ b/drivers/net/pasemi_mac.h
@@ -31,7 +31,7 @@
 	struct pas_dma_xct_descr	*desc;
 	dma_addr_t	 dma;
 	unsigned int	 size;
-	unsigned int	 next_to_use;
+	unsigned int	 next_to_fill;
 	unsigned int	 next_to_clean;
 	struct pasemi_mac_buffer *desc_info;
 	char		 irq_name[10];  /* "eth%d tx" */