sun4i-emac.c: remove unnecessary branch

According to the current implementation of emac_rx, every arrived packet
will be processed in the while loop. So, there is no remain packet last
time. The skb_last field and this branch for dealing with it is
unnecessary.

Signed-off-by: Conley Lee <conleylee@foxmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>