ARM: OMAP: fix return value check in beagle_opp_init()

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

dpatch engine is used to auto generated this patch.
(https://github.com/weiyj/dpatch)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Kevin Hilman <khilman@ti.com>