net/e1000: remove MTU setting limitation

net/e1000: remove MTU setting limitation

Currently, if requested MTU is bigger than mbuf size and scattered
receive is not enabled, setting MTU to that value fails.

This patch allows setting this special MTU when device is stopped,
because scattered_rx will be re-configured during next port start
and driver may enable scattered receive according new MTU value.

After this patch, driver may select different receive function
automatically after MTU set, according MTU values selected.

Fixes: 59d0ecdbf0e1 ("ethdev: MTU accessors")
Cc: stable@dpdk.org
Signed-off-by: Dapeng Yu <dapengx.yu@intel.com>
Acked-by: Jeff Guo <jia.guo@intel.com>