spi: fix initial SPI_SR value in spi-fsl-dspi

On ColdFire mcf54418, using DSPI_DMA_MODE mode, spi transfers
at first boot stage are not succeding:

m25p80 spi0.1: unrecognized JEDEC id bytes: 00, 00, 00

The reason is the SPI_SR initial value set by the driver, that
is not clearing (not setting to 1) the RF_DF flag. After a tour
on the dspi hw modules that use this driver(Vybrid, ColdFire and
ls1021a) a better init value for SR register has been set.
Signed-off-by: Angelo Dureghello <angelo@sysam.it>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-fsl-dspi.c

@@ -67,7 +67,7 @@
 #define SPI_SR			0x2c
 #define SPI_SR_EOQF		0x10000000
 #define SPI_SR_TCFQF		0x80000000
-#define SPI_SR_CLEAR		0xdaad0000
+#define SPI_SR_CLEAR		0x9aaf0000
 
 #define SPI_RSER_TFFFE		BIT(25)
 #define SPI_RSER_TFFFD		BIT(24)