korina: fix loop back of receive descriptors

After the last loop iteration, i has the value RC32434_NUM_RDS and
therefore leads to an index overflow when used afterwards to address the
last element. This is yet another another bug introduced when rewriting
parts of the driver for upstream preparation, as the original driver
used 'RC32434_NUM_RDS - 1' instead.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>