net: drivers: use bool type instead of double negation

Save some punctuation by using bool type's property equivalent to
doubled negation operator.

Reported-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>