PM / QoS: return -EINVAL for bogus strings

In the current code, if the user accidentally writes a bogus command to
this sysfs file, then we set the latency tolerance to an uninitialized
variable.

Fixes: 2d984ad1 (PM / QoS: Introcuce latency tolerance device PM QoS type)
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Cc: 3.15+ <stable@vger.kernel.org> # 3.15+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>