selftests: forwarding: ethtool: Move different_speeds_get() to ethtool_lib

Currently different_speeds_get() is used only by ethtool.sh tests.
The function can be useful for another tests that check ethtool
configurations.

Move the function to ethtool_lib in order to allow other tests to use
it.

Signed-off-by: Amit Cohen <amitc@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>