of_unflatten_dtb(): Check return value with IS_ERR
Of_unflatten_dtb returns a ERR_PTR value so checking it against NULL is
incorrect. Fix it in all of the places where this was happening.
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>