char/serial: fix copy&paste error (fifo8_is_full vs empty)

Copy&paste error in serial.c causes a crash when attempting
to read from UART (if there is no data to be read)
Signed-off-by: NVladimir Senkov <hangup@gmail.com>
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/char/serial.c

@@ -424,7 +424,7 @@ static uint64_t serial_ioport_read(void *opaque, hwaddr addr, unsigned size)
             ret = s->divider & 0xff;
         } else {
             if(s->fcr & UART_FCR_FE) {
-                ret = fifo8_is_full(&s->recv_fifo) ?
+                ret = fifo8_is_empty(&s->recv_fifo) ?
                             0 : fifo8_pop(&s->recv_fifo);
                 if (s->recv_fifo.num == 0) {
                     s->lsr &= ~(UART_LSR_DR | UART_LSR_BI);