of: fdt: Fix return with value in void function

of: fdt: Fix return with value in void function

Commit 49e67dd17649 "of: fdt: add missing allocation-failure check"
added a "return NULL" statement in __unflatten_device_tree().  When
applied to the 3.2-stable branch, this introduced a compiler warning
(not an error!) because the function returns void here.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>