drivers/net/r6040.c: fix sparse warning: Using plain integer as NULL pointer

Fix this sparse warnings:
  drivers/net/r6040.c:487:31: warning: Using plain integer as NULL pointer
  drivers/net/r6040.c:492:31: warning: Using plain integer as NULL pointer

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>