ixgbevf: Update ixgbevf_alloc_rx_buffers to handle clearing of status bits

Instead of clearing the status bits in the cleanup it makes more sense to
just clear the status bits on allocation.  This way we can leave the Rx
descriptor rings as a read only memory block until we actually have buffers
to give back to the hardware.

CC: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>