staging:rtl8192u: Correct comparison with NULL - Style

Correct code to remote comparison with NULL, this clears the resulting
checkpatch issue.

This is a coding style change which should not impact runtime code
execution.

Signed-off-by: John Whitmore <johnfwhitmore@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>