net: arc_emac: enable tx interrupts

In the current implementation the cleaning of tx ring is done by the
NAPI poll handler, which is scheduled after rx interrupts. Thus, in
absence of received packets the reclaim of used tx buffers is never
executed, blocking further transmission.

This can be easily reproduced starting the transmission of a UDP flow
with iperf, which blocks almost immediately because skbs are not
returned to the stack and the socket send buffer becomes full.

The patch enables tx interrupts so that the tx reclaim is scheduled
after completed transmissions.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>