dpaa2-switch: print an error when the vlan is already configured

Print a netdev error when we hit a case in which a specific VLAN is
already configured on the port. While at it, change the already existing
netdev_warn into an _err for consistency purposes.

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>