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

Clean up comparsions to NULL in the os_dep 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/20210730130204.18229-1-straube.linux@gmail.com


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