staging: rtl8712: Rewrite NULL comparisons

Replace NULL comparisons with boolean negation to be more consistent with
the rest of the Linux kernel code base.
Reported by checkpatch.

Signed-off-by: Zhansaya Bagdauletkyzy <zhansayabagdaulet@gmail.com>
Link: https://lore.kernel.org/r/74471865b399584d73a18696d2008006301dfd71.1617708653.git.zhansayabagdaulet@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>