e1000: remove risky prefetch on next_skb->data

It was brought to our attention that the prefetches break e1000 traffic
on xscale/arm architectures.  Remove them for now.  We'll let them
stay in mm for a while, or find a better solution to enable.

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