ARM: OMAP2+: PM: fix return value check in omap2_set_init_voltage()

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

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

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>