arm: omap2: io: fix clk_get() error check

clk_get() return value should be checked with IS_ERR().

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>