mlxsw: Enable string TLV usage according to MGIR output

String TLV is not supported by old firmware versions, therefore
'struct mlxsw_core' stores the field 'emad.enable_string_tlv', which is
set to true only after firmware version check.

Instead of assuming that firmware version check is enough to enable
string TLV, a better solution is to query if this TLV is supported from
MGIR register. Add such query and initialize 'emad.enable_string_tlv'
accordingly.

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>