ionic: remove callback pointer from desc_info

By reworking the queue service routines to have their own
servicing loops we can remove the cb pointer from desc_info
to save another 8 bytes per descriptor,

This simplifies some of the queue handling indirection and makes
the code a little easier to follow, and keeps service code in
one place rather than jumping between code files.

   struct ionic_desc_info
	Before:  /* size: 472, cachelines: 8, members: 7 */
	After:   /* size: 464, cachelines: 8, members: 6 */

Suggested-by: Neel Patel <npatel2@amd.com>
Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>