We already have NMEthtoolID to handle coalesce options in a way that is
convenient programmatically. That is, we can iterate over all valid
coalesce options (it's just an integer) and use that in a more generic
way.

If NMEthtoolCoalesceState names all fields explicitly, we need explicit
code that names each coalesce option. Especially since NMEthtoolCoalesceState
is an internal and intermediate data structure, this is cumbersome
and unnecessary.

Thereby it also fixes the issue that nm_platform_ethtool_init_coalesce() has a
NMPlatform argument without actually needing it.
nm_platform_ethtool_init_coalesce() does not operate on a NMPlatform
instance, and should not have the appearance of being a method of
NMPlatform.