switchdev: fix BUG when port driver doesn't support set attr op

Fix a BUG_ON() where CONFIG_NET_SWITCHDEV is set but the driver for a
bridged port does not support switchdev_port_attr_set op.  Don't BUG_ON()
if -EOPNOTSUPP is returned.

Also change BUG_ON() to netdev_err since this is a normal error path and
does not warrant the use of BUG_ON(), which is reserved for unrecoverable
errs.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Reported-by: Brenden Blanco <bblanco@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>