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: NAmit Cohen <amcohen@nvidia.com>
Reviewed-by: NPetr Machata <petrm@nvidia.com>
Signed-off-by: NIdo Schimmel <idosch@nvidia.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>