ice: add missing WRITE_ONCE when clearing ice_rx_ring::xdp_prog

ice: add missing WRITE_ONCE when clearing ice_rx_ring::xdp_prog

It is read by data path and modified from process context on remote cpu
so it is needed to use WRITE_ONCE to clear the pointer.

Fixes: efc2214b6047 ("ice: Add support for XDP")
Reviewed-by: Shannon Nelson <shannon.nelson@amd.com>
Tested-by: Chandan Kumar Rout <chandanx.rout@intel.com> (A Contingent Worker at Intel)
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>