ixgbe: cleanup ixgbe_clean_rx_irq

The code for ixgbe_clean_rx_irq was much more tangled up than it needed to
be in terms of logic statements and unused variables.  This change
untangles much of that and drops several unused variables such as cleaned
which was being returned but never checked.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>