r6040: use ETH_ZLEN instead of MISR for SKB length checking

Ever since this driver was merged the following code was included:

if (skb->len < MISR)
	skb->len = MISR;

MISR is defined to 0x3C which is also equivalent to ETH_ZLEN, but use
ETH_ZLEN directly which is exactly what we want to be checking for.

Reported-by: Marc Volovic <marcv@ezchip.com>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>