ixgbe: Drop Rx alloc at end of Rx cleanup

This change removes the Rx buffer allocation at the end of ixgbe_clean_rx_irq.
The reason for removing this is to avoid the extra latency introduced by the
MMIO write.  This can amount to somewhere around an extra 100ns of latency and
one extra message worth of PCIe bus overhead.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>