mlxsw: core: Do not worry about changing 'enable_string_tlv' while sending EMADs

Till now, the field 'mlxsw_core->emad.enable_string_tlv' is set as part
of mlxsw_sp_init(), this means that it can be changed during
emad_reg_access(). To avoid such change, this field is read once in
emad_reg_access() and the value is used all the way.

The previous patch sets this value according to MGIR output, as part of
mlxsw_emad_init(), so now it cannot be changed while sending EMADs.

Do not save 'enable_string_tlv' and do not pass it to functions, just pass
'struct mlxsw_core' and use the value directly from it.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>