ARM: shmobile: koelsch: fix error return code check from clk_get()

The koelsch_add_standard_devices() function calls clk_get() but then fails
to check that it returns an error pointer instead of NULL on failure.

This was added by f31239ef ("ARM: shmobile: koelsch-reference:
Instantiate clkdevs for SCIF and CMT") in Simon Horman's
renesas-boards2-for-v3.14 tag.

The issue is not serious as it does not cause a crash and seems to not be
actually causing any issues now the other clock bugs have been fixed.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[horms+renesas@verge.net.au: tweaked changelog]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>