net: stmmac: dwmac4: Re-enable MAC Rx before powering down

Re-enable the MAC receiver by setting CONFIG_RE before powering down,
as instructed in section 6.3.5.1 of [1].  Without this the MAC fails
to receive WoL packets and never wakes up.

[1] DWC Ethernet QoS Databook 4.10a October 2014

Signed-off-by: Ed Blake <ed.blake@sondrel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>