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

In case of error, the function omap_device_alloc() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check
should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>