e1000: fix panic on large frame receive when mtu=default

A panic was reported when receiving 1522 size packets if using
the default MTU. we should set the initial rx buffer length to the
value that e1000changemtu sets so that we can receive any packet
that would not be dropped by LPE=0.

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