cnic: Explicitly initialize all reference counts to 0.

The driver is relying on zero'ed allocated memory and does not
explicitly call atomic_set() to initialize the ref counts to 0.  Add
these atomic_set() calls so that it will be more straight forward
to convert atomic ref counts to refcount_t.

Reported-by: Chuhong Yuan <hslester96@gmail.com>
Cc: Rasesh Mody <rmody@marvell.com>
Cc: <GR-Linux-NIC-Dev@marvell.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>