net: Add ethtool to mii advertisment conversion helpers

Translating between ethtool advertisement settings and MII
advertisements are common operations for ethernet drivers.  This patch
adds a set of helper functions that implements the conversion.  The
patch then modifies a couple of the drivers to use the new functions.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>