staging: r8188eu: clean up comparsions to NULL in core directory

Clean up comparsions to NULL in the core directory reported by
checkpatch.

x == NULL -> !x
x != NULL -> x

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20210801084614.4328-2-straube.linux@gmail.com


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