staging: rtl8188eu: clean up comparsions to NULL

Clean up remaining comparsions to NULL reported by checkpatch.
x == NULL -> !x
x != NULL -> x

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20200929062847.23985-2-straube.linux@gmail.com


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