drivers: eth_sam_gmac: Fix 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 db11fcd1 "net/net_pkt: Fully separate struct
net_pkt from struct net_buf".

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>