net: prestera: replace zero-length array with flexible-array member

One-element and zero-length arrays are deprecated and should be
replaced with flexible-array members:
https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

Replace zero-length array with flexible-array member and make use
of the struct_size() helper.

Link: https://github.com/KSPP/linux/issues/78


Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Volodymyr Mytnyk <vmytnyk@marvell.com>
Tested-by: Volodymyr Mytnyk <vmytnyk@marvell.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20211204171349.22776-1-jose.exposito89@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>