ice: Clean control queues only when they are initialized

Clean control queues only when they are initialized. One of the ways to
validate if the basic initialization is done is by checking value of
cq->sq.head and cq->rq.head variables that specify the register address.
This patch adds a check to avoid NULL pointer dereference crash when tried
to shutdown uninitialized control queue.

Signed-off-by: Preethi Banala <preethi.banala@intel.com>
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>