spi_mpc83xx.c underclocking hotfix

The MPC83xx SPI controller clock divider can divide the system clock by not
more then 1024.  The spi_mpc83xx driver does not check this and silently
writes garbage to the SPI controller registers when asked to run at lower
frequencies.  I've tried to run the SPI on a 266MHz MPC8349E with 100kHz
for debugging a bus problem and suddenly was confronted with a 2nd problem
to debug..  ;-)

The patch adds an additional check which avoids writing garbage to the SPI
controller registers and warn the user about it.  This might help others to
avoid simmilar problems.

Cc: Kumar Gala <galak@gate.crashing.org>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/spi/spi_mpc83xx.c

@@ -158,6 +158,12 @@ static void mpc83xx_spi_chipselect(struct spi_device *spi, int value)
 
 		if ((mpc83xx_spi->sysclk / spi->max_speed_hz) >= 64) {
 			u8 pm = mpc83xx_spi->sysclk / (spi->max_speed_hz * 64);
+			if (pm > 0x0f) {
+				printk(KERN_WARNING "MPC83xx SPI: SPICLK can't be less then a SYSCLK/1024!\n"
+						"Requested SPICLK is %d Hz. Will use %d Hz instead.\n",
+						spi->max_speed_hz, mpc83xx_spi->sysclk / 1024);
+				pm = 0x0f;
+			}
 			regval |= SPMODE_PM(pm) | SPMODE_DIV16;
 		} else {
 			u8 pm = mpc83xx_spi->sysclk / (spi->max_speed_hz * 4);