net: dsa: felix: fix flexible_array.cocci warnings

Zero-length and one-element arrays are deprecated, see
Documentation/process/deprecated.rst
Flexible-array members should be used instead.

Generated by: scripts/coccinelle/misc/flexible_array.cocci

Fixes: 23ae3a78 ("net: dsa: felix: add stream gate settings for psfp")
CC: Xiaoliang Yang <xiaoliang.yang_1@nxp.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: kernel test robot <lkp@intel.com>
Signed-off-by: Julia Lawall <julia.lawall@inria.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>