net: stmmac: restructure tc implementation for RX VLAN Priority steering

The current tc_add_flow() and tc_del_flow() use hardware L3 & L4 filters
as offloading. The number of L3/L4 filters is read from L3L4FNUM field
from MAC_HW_Feature1 register and is used to alloc priv->tc_entries[].

For RX frame steering based on VLAN priority offloading, we use
MAC_RXQ_CTRL2 & MAC_RXQ_CTRL3 registers and all VLAN priority level
can be configured independent from L3 & L4 filters.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>