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

The lager_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 4a606af2 ("ARM: shmobile: lager-reference: Instantiate
clkdevs for SCIF and CMT") patch 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>