net: ethernet: ti: cpdma: correct error handling for chan create

It's not correct to return NULL when that is actually an error and
function returns errors in any other wrong case. In the same time,
the cpsw driver and davinci emac doesn't check error case while
creating channel and it can miss actual error. Also remove WARNs
replacing them on dev_err msgs.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>