This change adds a mutex to serialize accesses of sysfs attributes.

This is required when polling CVRF bit of the MASK/ENABLE register
because this bit is cleared on a read of this MASK/ENABLE register
or a write to CONFIG register, which means that this bit might be
accidentally cleared by reading other fields like alert flags.

So this patch adds a mutex lock to protect the write() and read()
callbacks. The read_string() callback won't need the lock since it
just returns the label without touching any hardware register.
Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>