staging: rtl8192u: rewrite NULL comparison for pointers

When testing pointers, it is not necessary to explicitly compare to
NULL. Rewrite if condition as (!ptr) or (ptr) as suggested in
Documentation/CodingStyle

Signed-off-by: Nicholas Sim <nicholassimws@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>