net: renesas: rswitch: Fix the output value of quote from rswitch_rx()

If the RX descriptor doesn't have any data, the output value of quote
from rswitch_rx() will be increased unexpectedily. So, fix it.

Reported-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Fixes: 3590918b ("net: ethernet: renesas: Add support for "Ethernet Switch"")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>