net: ethernet: ti: am65-cpsw: Fix error code in am65_cpsw_nuss_init_tx_chns()

This accidentally returns success, but it should return a negative error
code.

Fixes: 93a76530

 ("net: ethernet: ti: introduce am65x/j721e gigabit eth subsystem driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>