ethtool: pause: make sure we init driver stats

The intention was for pause statistics to not be reported
when driver does not have the relevant callback (only
report an empty netlink nest). What happens currently
we report all 0s instead. Make sure statistics are
initialized to "not set" (which is -1) so the dumping
code skips them.

Fixes: 9a27a330 ("ethtool: add standard pause stats")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>