clk: socfpga: Silence sparse warning

drivers/clk/socfpga/clk-periph.c:79:39: warning: Using plain integer as NULL pointer

Cc: Dinh Nguyen <dinguyen@altera.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
diff --git a/drivers/clk/socfpga/clk-periph.c b/drivers/clk/socfpga/clk-periph.c
index 46531c3..83aeaa2 100644
--- a/drivers/clk/socfpga/clk-periph.c
+++ b/drivers/clk/socfpga/clk-periph.c
@@ -76,7 +76,7 @@
 		periph_clk->shift = div_reg[1];
 		periph_clk->width = div_reg[2];
 	} else {
-		periph_clk->div_reg = 0;
+		periph_clk->div_reg = NULL;
 	}
 
 	rc = of_property_read_u32(node, "fixed-divider", &fixed_div);