HID: picoLCD: prevent NULL pointer dereferences

Driver code expects to get access to struct picolcd_data from hiddev
and is not prepared to find a NULL pointer there. Most prominent
candidate to trip on it is picolcd_fb_deferred_io().
Delay removing struct picolcd_data from hiddev until all sub-devices
have been unregistered.

Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>