sparc64: Add vio_set_intr() to enable/disable Rx interrupts

The vio_set_intr() API should be used by VIO consumers to enable/disable
Rx interrupts to facilitate deferred processing in softirq/bottom-half
context.

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>