ixgbe: cleanup some register reads

Remove duplicate inc of hwstats->ruc
Introduce separate loops for 8 and 16 register reads.
Consolidate mac checks under one case.
Make sure registers are cleared on read.

Reported-by: Jonathan Lynch <jonathan.lynch@thenowfactory.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
CC: Jonathan Lynch <jonathan.lynch@thenowfactory.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>