sb1250-mac: duplicate setting of bit V_MAC_RX_PL_THRSH(4) in sbmac_channel_start()

The bit V_MAC_RX_PL_THRSH(4) was already set a few lines higher.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>