e1000: Remove unnecessary usages of memset

Replace memsets of 1 byte with simple assignments.
Issue reported by checkpatch.pl.

Signed-off-by: Suraj Upadhyay <usuraj35@gmail.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>