net: stmmac: Let TX and RX interrupts be independently enabled/disabled

By using this mechanism we can get rid of the not so nice method of
scheduling TX NAPI when the RX was scheduled. No bandwidth reduction was
seen with this change.

Changes from v1:
- Remove useless comment (Jakub)
- Do not bind the TX clean to NAPI budget (Jakub)

Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>