flags used within drivers for indicating tx and rx flow control are
defined in 4 drivers (and probably more), move these constants to mii.h.

The 3 SMSC drivers use the same constants (FLOW_CTRL_TX), but TG3 uses
TG3_FLOW_CTRL_TX, so this patch also renames the constants within TG3.
Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>