net/e1000/base: fall through explicitly

net/e1000/base: fall through explicitly

Found some inconsistent code comments when it came to when we "fall
through", so made them more consistent and non-repetitive.

This patch adds/changes fall through comments to address new warnings
produced by gcc 7.

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Todd Fujinaka <todd.fujinaka@intel.com>
Signed-off-by: Guinan Sun <guinanx.sun@intel.com>
Reviewed-by: Wei Zhao <wei.zhao1@intel.com>