In case of error, the function omap_device_alloc() 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: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NTony Lindgren <tony@atomide.com>