mlxsw: core: Add support for using EMAD string TLV

In case the firmware had an error while processing EMADs, it can send back
an ASCII string with the reason using EMAD string TLV.

This patch adds the support for using EMAD string TLV. In case of an error,
reports the reason using devlink hwerr tracepoint.

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>