sfc: Make buffer table indices and counts consistently unsigned

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/ethernet/sfc/net_driver.h b/drivers/net/ethernet/sfc/net_driver.h
index f526877..59c0583 100644
--- a/drivers/net/ethernet/sfc/net_driver.h
+++ b/drivers/net/ethernet/sfc/net_driver.h
@@ -81,8 +81,8 @@
 	void *addr;
 	dma_addr_t dma_addr;
 	unsigned int len;
-	int index;
-	int entries;
+	unsigned int index;
+	unsigned int entries;
 };
 
 /**