MIPS: ath79: Fix error handling

'clk_register_fixed_rate()' returns an error pointer in case of error, not
NULL. So test it with IS_ERR.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Aban Bedel <albeu@free.fr>
Cc: antonynpavlov@gmail.com
Cc: hackpascal@gmail.com
Cc: amitoj1606@gmail.com
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel-janitors@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14464/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/ath79/clock.c

@@ -45,7 +45,7 @@ static struct clk *__init ath79_add_sys_clkdev(
 	int err;
 
 	clk = clk_register_fixed_rate(NULL, id, NULL, 0, rate);
-	if (!clk)
+	if (IS_ERR(clk))
 		panic("failed to allocate %s clock structure", id);
 
 	err = clk_register_clkdev(clk, id, NULL);