ionic: check port ptr before use

Check for corner case of port_init failure before using
the port_info pointer.

Fixes: 4d03e00a

 ("ionic: Add initial ethtool support")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Link: https://lore.kernel.org/r/20201104195606.61184-1-snelson@pensando.io
Signed-off-by: Jakub Kicinski <kuba@kernel.org>