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

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>