MIPS: ath79: Fix error handling

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>