hw/ssi/pl022: Correct wrong value for PL022_INT_RT

hw/ssi/pl022: Correct wrong value for PL022_INT_RT

The PL022 interrupt registers have bits allocated as:
 0: ROR (receive overrun)
 1: RT (receive timeout)
 2: RX (receive FIFO half full or less)
 3: TX (transmit FIFO half full or less)

A cut and paste error meant we had the wrong value for
the PL022_INT_RT constant. This bug doesn't affect device
behaviour, because we don't implement the receive timeout
feature and so never set that interrupt bit.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180820141116.9118-20-peter.maydell@linaro.org
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>