KS8695: update ksp->next_rx_desc_read at the end of rx loop

There is no need to adjust the next rx descriptor after each packet,
so do it only once at the end of the routine.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>