e1000: FIX: Disable Packet Split for non jumbo frames

Allocations using alloc_page are taking too long for normal MTU, so
use LPE only for jumbo frames.

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