mlxsw: spectrum_span: Wrap buffer change in a function

The code that adjusts the egress buffer size is not symmetric at the
moment. The update is done via a call to
mlxsw_sp_span_port_buffer_update(), but the disablement is done inline
by invoking the write to SBIB register directly.

Wrap the disablement code in mlxsw_sp_span_port_buffer_disable().

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