cnic: Add functions to allocate and free UIO rings

These functions are needed to free up memory when the rings are no longer
needed.

Reviewed-by: Eddie Wai <eddie.wai@broadcom.com>
Reviewed-by: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>