i40e: fix erroneous WARN_ON

The driver was issuing a WARN_ON during ring size changes
because the code was cloning the rx_ring struct but
not zeroing out the pointers before allocating new memory.

Zero out the pointers in the cloned copy before allocating
new memory for them.  In this case the code was correctly
avoiding memory leaks but still triggering the warning.

Change-ID: I186dd493948e9b7254ab0593d4aad8b68808918d
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>