net: sh_eth: Split sh_eth_recv

Split sh_eth_recv into two functions, one which checks whether
a packet was received and one which handles the received packet.
This is done in preparation for DM support, which handles these
two parts separately.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Joe Hershberger <joe.hershberger@ni.com>