net: sh_eth: Fix timing of RACT setting in sh_eth_rx()

This patch fixes an issue that we cannot use nfs rootfs correctly
on r8a7790 when the command below runs on a host PC.

 $ sudo ping -f -l 8 $BOARD_IP_ADDR

Since the driver sets the RACT to 1 in the first while loop of
sh_eth_rx(), the controller accepts a next frame into the next RX
descriptor during the while loop. But, in the first while loop
doesn't allocate a next skb. So, this patch removes the RACT setting
in the first while loop of sh_eth_rx().

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>