mlxsw: core: Create the mlxsw_fw_rev struct

This struct was previously an anonymous struct defined inside the
mlxsw_bus_info struct. Extract it to a struct named mlxsw_fw_rev, as it
will be needed later by the spectrum driver.

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