net: dsa: loop: Fix uninitialized pvid variable

Dan's static analyzer reported the following:

	drivers/net/dsa/dsa_loop.c:181 dsa_loop_port_vlan_del()
	error: XXX uninitialized symbol 'pvid'.

we were missing the assignment of pvid to ps->vid, so add that.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 98cd1552 ("net: dsa: Mock-up driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>