net: ethernet: ti: davinci_cpdma: split descs num between all channels

Tx channels share same pool of descriptors. Thus one channel can
block another if pool is emptied by one. But, the shaper should
decide which channel is allowed to send packets. To avoid such
impact of one channel on another, let every channel to have its
own piece of pool.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>