net: spacemit: Avoid -Wflex-array-member-not-at-end warnings

net: spacemit: Avoid -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use regular arrays instead of flexible-array members (they're not
really needed in this case) in a couple of unions, and fix the
following warnings:

      1 drivers/net/ethernet/spacemit/k1_emac.c:122:42: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
      1 drivers/net/ethernet/spacemit/k1_emac.c:122:32: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
      1 drivers/net/ethernet/spacemit/k1_emac.c:121:42: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
      1 drivers/net/ethernet/spacemit/k1_emac.c:121:32: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Vivian Wang <wangruikang@iscas.ac.cn>
Link: https://patch.msgid.link/aPd0YjO-oP60Lgvj@kspp
Signed-off-by: Jakub Kicinski <kuba@kernel.org>