ARM: OMAP3: Beagle: fix return value check in beagle_opp_init()

In case of error, the function get_cpu_device() returns NULL pointer
not ERR_PTR(). The IS_ERR() test in the return value check should be
replaced with NULL test.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Tony Lindgren <tony@atomide.com>