HID: hid-picolcd: don't use flush_scheduled_work()

flush_scheduled_work() is deprecated and scheduled to be removed.
Directly flush picolcd_fb_cleanup on exit instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>