staging: rtl8192u: Replace explicit NULL comparisons with !

This patch replace explicit NULL comparison with ! or unmark operator to
simplify code.
Reported by checkpatch.pl for comparison to NULL could be
written "!XXX" or "XXX".

Signed-off-by: simran singhal <singhalsimran0@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>