net: ethtool: add helpers for MM fragment size translation

We deliberately make the Linux UAPI pass the minimum fragment size in
octets, even though IEEE 802.3 defines it as discrete values, and
addFragSize is just the multiplier. This is because there is nothing
impossible in operating with an in-between value for the fragment size
of non-final preempted fragments, and there may even appear hardware
which supports the in-between sizes.

For the hardware which just understands the addFragSize multiplier,
create two helpers which translate back and forth the values passed in
octets.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>