mfd: AB8500 mask off irrelevant bits from the SPI message

The registers on the AB8500 are only 8 bits wide, so the content
of the remaining bits is undefined. Let's mask off the undefined
stuff when returning a register in an SPI read.

Acked-by: Rabin Vincent <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>
Signed-off-by: Samuel Ortiz <[e-mail]>

drivers/mfd/ab8500-spi.c

···
 
 	ret = spi_sync(spi, &msg);
 	if (!ret)
-		ret = ab8500->rx_buf[0];
+		/*
+		 * Only the 8 lowermost bytes are
+		 * defined with value, the rest may
+		 * vary depending on chip/board noise.
+		 */
+		ret = ab8500->rx_buf[0] & 0xFFU;
 
 	return ret;
 }