mlxsw: spectrum: Publish mlxsw_sp_ethtype_to_sver_type()

Declare mlxsw_sp_ethtype_to_sver_type() in spectrum.h to enable using it
in other files.

It will be used in the next patch to map between EtherType and the
relevant value configured by SVER register.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>