net: stmmac: dwxgmac2: Also pass control frames while in promisc mode

In order for the selftests to run the Flow Control selftest we need to
also pass pause frames to the stack.

Pass this type of frames while in promiscuous mode.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>