net/mlx5e: Fix port buffer function documentation format

This patch fixes compiler warnings:
In drivers/.../mlx5/core/en/port_buffer.c:190:
warning: Function parameter or member 'pfc_en' not described...
...
warning: Function parameter or member 'change' not described...

Fixes: 0696d608 ("net/mlx5e: Receive buffer configuration")
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>