ixgbevf: have clean_rx_irq return total_rx_packets cleaned

Rather than return true/false indicating whether there was budget left, return
the total packets cleaned. This currently has no use, but will be used in a
following patch which enables CONFIG_NET_RX_BUSY_POLL support in order to track
how many packets were cleaned during the busy poll as part of the extended
statistics.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>