net: systemport: rename rx_csum_en to rx_chk_en

This boolean tells us whether we are using the RXCHK hardware block,
so use a variable name that reflects that. RXCHK might be used in the
future to implement Wake-on-LAN using ARP or unicast packets.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>