ARM: OMAP: omap_device: fix return value check in omap_device_build_ss()

In case of error, the function omap_device_alloc() returns ERR_PTR()
and never returns 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>