net: ethernet: ti: davinci_cpdma: move cpdma channel struct macroses to internals

Keep the driver internals in C file. Currently it's not required for
drivers to know rx or tx a channel is, except create function.
So correct "channel create" function, and use all channel struct
macroses only for internal use.

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