sfc: Make buffer table indices and counts consistently unsigned

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/ethernet/sfc/nic.c b/drivers/net/ethernet/sfc/nic.c
index 1bb41ff..2bdfb63 100644
--- a/drivers/net/ethernet/sfc/nic.c
+++ b/drivers/net/ethernet/sfc/nic.c
@@ -192,7 +192,7 @@
 efx_init_special_buffer(struct efx_nic *efx, struct efx_special_buffer *buffer)
 {
 	efx_qword_t buf_desc;
-	int index;
+	unsigned int index;
 	dma_addr_t dma_addr;
 	int i;