wifi: iwlwifi: dbg-tlv: use struct_size() for allocation

wifi: iwlwifi: dbg-tlv: use struct_size() for allocation

This is effectively the same since we don't even have a
multiplication, but better captures what happens with
the length.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240128084842.f301641eb916.I66b7b48a526377d682eecef874373bf3a01076c8@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>