smsc911x: define status word positions as constants

The vast majority of bit constants in this driver are defined in the
header file, but TX and RX status word bits are not.  This patch (which
should make no functional change) defines these, to make the driver
slightly more readable.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>