igb is currently not returning the correct values for napi.  In addition it
is doing more work than necessary since it will not exit polling until
work_done is equal to zero.

This patch makes the following changes:
1.  Consolidates msi-x and non-msi polling routines.
2.  Corrects return values for polling routines.
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>