drivers: net: cpsw: add separate napi for tx

Instead of processing tx events in isr adding separate napi for
tx which improves performance by ~180Mbps with
omap2plus_defconfig on DRA74x platform. Also cleaning up rx napis
by renaming to napi_rx for better understanding the code.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>