of: overlay.c: Convert comparisons to zero or NULL to logical expressions

Use normal shorthand for comparing a variable to zero.
For variable "XXX":
   convert (XXX == 0) to (!XXX)
   convert (XXX != 0) to (XXX)

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