staging: dpaa2-switch: make sure that the VLAN is not already configured

staging: dpaa2-switch: make sure that the VLAN is not already configured

When in the PREPARE state of a switchdev transaction, check if the
requested VLAN is not already configured on the switch port. This keeps
us from getting into a WARNING as below.

[ 1389.683296] dpaa2_ethsw dpsw.0 eth0: VLAN 2 already configured
[ 1389.689125] ------------[ cut here ]------------
[ 1389.694084] eth0: Commit of object (id=1) failed.
[ 1389.698863] WARNING: CPU: 0 PID: 613 at net/switchdev/switchdev.c:277 switchdev_port_obj_add_now+0xcc/0x110
[ 1389.708589] Modules linked in:
[ 1389.711634] CPU: 0 PID: 613 Comm: bridge Not tainted 5.9.0-rc2-next-20200828-00112-g7172078477c5 #59
[ 1389.720753] Hardware name: NXP Layerscape LX2160ARDB (DT)
[ 1389.726139] pstate: 40000005 (nZcv daif -PAN -UAO BTYPE=--)
[ 1389.731698] pc : switchdev_port_obj_add_now+0xcc/0x110
[ 1389.736824] lr : switchdev_port_obj_add_now+0xcc/0x110

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>