net: atlantic: 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.

Remove unused flexible-array member `buf` and, with this, fix the following
warnings:
drivers/net/ethernet/aquantia/atlantic/aq_hw.h:197:36: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/net/ethernet/aquantia/atlantic/hw_atl/../aq_hw.h:197:36: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Suggested-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Igor Russkikh <irusskikh@marvell.com>
Link: https://patch.msgid.link/Z6F3KZVfnAZ2FoJm@kspp


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