We should not enter close function while someone else is in open.
This mutex prevents this race.

There is also no need to override the ret value with -EIO in case of
a failure of i2c_hid_set_power.
Signed-off-by: NBenjamin Tissoires <benjamin.tissoires@gmail.com>
Reviewed-by: NJean Delvare <khali@linux-fr.org>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>