drivers/eth/mcux: Fix the fragment ordering in RX

The data fragments were stored in reversed order when the RX
data was saved into network buffers. This was caused by net_pkt
changes in commit "net/net_pkt: Fully separate struct net_pkt
from struct net_buf".

Change-Id: I8ad2cfc23b2cb90896b0548eab168895b0d7421d
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>