stmmac: Enable unicast pause frame detect in GMAC Register 6

Unicast pause frame detect was not being enabled for the Synopsys stmmac. This
patch sets Unicast pause frame detect in MAC register 6 so that pause frame
detection by the stmmac conforms to IEEE 802.3, Annex 31B.3.3 Receive
Operation - Specifically, a MAC shall respond to pause frames containing
either the reserved multicast address or the unique physical address
associated with this station.

Signed-off-by: Vince Bridgers <vbridger@opensource.altera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>