e1000: Remove unneeded and unwanted memsets

This memsetting was added in a paranoid rage debugging TX hangs, but
are no longer of importance. We can beef up the performance quite a
bit removing them. Make sure to fill in next_to_watch to allow this.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>