hwmon: (max1111) Fix race condition causing NULL pointer exception

spi_sync call uses its spi_message parameter to keep completion information,
using a drvdata structure is not thread-safe. Use a mutex to prevent
multiple access to shared driver data.

Signed-off-by: Pavel Herrmann <morpheus.ibis@gmail.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Cyril Hrubis <metan@ucw.cz>
Tested-by: Stanislav Brabec <utx@penguin.cz>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: stable@kernel.org