HID: thingm: improve locking

Reading from the device consists of two operations: sending the read
command and the actual read from the device. If the device is accessed
in between we might read wrong data. Therefore protect the full sequence
of both operations with a mutex.
Also change the semantics of thingm_recv to include both operations.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>