mlxsw: Query maximum number of ports from firmware

We currently hard code the maximum number of ports in the driver, but
this may change in future devices, so query it from the firmware
instead.

Fallback to a maximum of 64 ports in case this number can't be queried.
This should only happen in SwitchX-2 for which this number is correct.

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