net: wwan: core: Return poll error in case of port removal

Ensure that the poll system call returns proper error flags when port
is removed (nullified port ops), allowing user side to properly fail,
without further read or write.

Fixes: 9a44c1cc ("net: Add a WWAN subsystem")
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>