sh: boards: mach-se: use IS_ERR() instead of NULL check

clk_get() returns ERR_PTR() on error, not NULL.

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
Reviewed-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>