mlxsw: core_linecards: Implement line card activation process

Allow to process events generated upon line card getting "ready" and
"active".

When DSDSC event with "ready" bit set is delivered, that means the
line card is powered up. Use MDDC register to push the line card to
active state. Once FW is done with that, the DSDSC event with "active"
bit set is delivered.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>