net: ethtool: export conversion function between u32 and link mode

The function convert_legacy_u32_to_link_mode and
convert_link_mode_to_legacy_u32 may be used outside
of ethtool.c. We rename them to ethtool_convert_...
and export them, so we could use them in others
drivers and modules.

Signed-off-by: Philippe Reynes <tremyfr@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>