spi/fsl_espi: change the read behaviour of the SPIRF

The user must read N bytes of SPIRF (1 <= N <= 4) that do not exceed the
amount of data in the receive FIFO, so read the SPIRF byte by byte when
the data in receive FIFO is less than 4 bytes.

On Simics, when read N bytes that exceed the amout of data in receive
FIFO, we can't read the data out, that is we can't clear the rx FIFO,
then the CPU will loop on the espi rx interrupt.
Signed-off-by: Mingkai Hu <Mingkai.hu@freescale.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/spi_fsl_espi.c

@@ -507,16 +507,29 @@ void fsl_espi_cpu_irq(struct mpc8xxx_spi *mspi, u32 events)
 
 	/* We need handle RX first */
 	if (events & SPIE_NE) {
-		u32 rx_data;
+		u32 rx_data, tmp;
+		u8 rx_data_8;
 
 		/* Spin until RX is done */
 		while (SPIE_RXCNT(events) < min(4, mspi->len)) {
 			cpu_relax();
 			events = mpc8xxx_spi_read_reg(&reg_base->event);
 		}
-		mspi->len -= 4;
 
+		if (mspi->len >= 4) {
 			rx_data = mpc8xxx_spi_read_reg(&reg_base->receive);
+		} else {
+			tmp = mspi->len;
+			rx_data = 0;
+			while (tmp--) {
+				rx_data_8 = in_8((u8 *)&reg_base->receive);
+				rx_data |= (rx_data_8 << (tmp * 8));
+			}
+
+			rx_data <<= (4 - mspi->len) * 8;
+		}
+
+		mspi->len -= 4;
 
 		if (mspi->rx)
 			mspi->get_rx(rx_data, mspi);