net: ethernet: ti: davinci_cpdma: don't stop ctlr if it was stopped

No need to stop ctlr if it was already stopped. It can cause timeout
warns. Steps:
- ifconfig eth0 down
- ethtool -l eth0 rx 8 tx 8
- ethtool -l eth0 rx 1 tx 1

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>