net/macmace: Replace zero-length array with DECLARE_FLEX_ARRAY() helper

net/macmace: Replace zero-length array with DECLARE_FLEX_ARRAY() helper

Since zero-length arrays are deprecated, we are replacing
them with C99 flexible-array members. As a result, instead
of declaring a zero-length array, use the new
DECLARE_FLEX_ARRAY() helper macro.

This fixes warnings such as:
./drivers/net/ethernet/apple/macmace.c:80:4-8: WARNING use flexible-array member instead (https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays)

Signed-off-by: Atul Raut <rauji.raut@gmail.com>
Link: https://lore.kernel.org/r/20230730231442.15003-1-rauji.raut@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>