igb: update napi polling to consolidate function and return correct values

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>