arm: plat-omap: counter_32k: use IS_ERR() instead of NULL check

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

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
[tony@atomide.com: updated to include err.h to compile on omap1]
Signed-off-by: Tony Lindgren <tony@atomide.com>