of: overlay: remove unneeded check for NULL kbasename()

kbasename() will not return NULL if passed a valid string.  If
the parameter passed to kbasename() in this case is already NULL
then the devicetree has been corrupted.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>