serial/nwpserial: Fix wrong register read address and add interrupt acknowledge.

serial/nwpserial: Fix wrong register read address and add interrupt acknowledge.

The receive interrupt routine checks the wrong register if the
receive fifo is empty. Further an explicit interrupt acknowledge
write is introduced. In some circumstances another interrupt was
issued.

Signed-off-by: Benjamin Krill <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>
Signed-off-by: Benjamin Herrenschmidt <[email protected]>