net/sfc: avoid usage of possibly uninitialized link mode

net/sfc: avoid usage of possibly uninitialized link mode

In fact efx_port_poll() always initializes it, but it isn't
explicitly documented feature of the API. Moreover, the API
annocation suggests that return code should be checked.

Fixes: 886f8d8a05bf ("net/sfc: retrieve link info")

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Andrew Lee <alee@solarflare.com>