ANDROID: hid: uhid: use ll_open_count for refcounting

Upstream commit 283a21da ("HID: remove no longer used hid->open field"),
removed obsolete hid->open, so use hid->ll_open_count to track refcount.
Also use existing hid->ll_open_lock mutex instead.

Fixes: Change-Id: I57413e42ec961a960a8ddc4942228df22c730d80
       ("ANDROID: hid: uhid: implement refcount for open and close")
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>