MIPS: ath79: Fix test for error return of clk_register_fixed_factor().

clk_register_fixed_factor returns an ERR_PTR in case of an error and
should have an IS_ERR check instead of a null check.

The Coccinelle semantic patch used to find this issue is as follows:
@@
expression e;
statement S;
@@

*e = clk_register_fixed_factor(...);
if (!e) S
Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Cc: julia.lawall@lip6.fr
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13894/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/ath79/clock.c

@@ -96,7 +96,7 @@ static struct clk * __init ath79_reg_ffclk(const char *name,
 	struct clk *clk;
 
 	clk = clk_register_fixed_factor(NULL, name, parent_name, 0, mult, div);
-	if (!clk)
+	if (IS_ERR(clk))
 		panic("failed to allocate %s clock structure", name);
 
 	return clk;