Bluetooth: Avoid a couple dozen -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 the __struct_group() helper to fix 31 instances of the following
type of warnings:

30 net/bluetooth/mgmt_config.c:16:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
1 net/bluetooth/mgmt_config.c:22:33: 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>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>