staging: r8723au: replace explicit NULL comparisons with !

Replace explicit NULL comparison with ! operator to simplify code.

Addresses checkpatch.pl:
WARNING: Comparisons should place the constant on the right side of the
test

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>