HID: Consolidate device existence checks in hiddev_ioctl

Currently, if the device has been removed before hiddev_ioctl(),
the -EIO is returned. If it's removed while hiddev_ioctl() is in
progress, some commands are still processed fine, others
return -ENODEV. This change takes the "existancelock" before
processing ioctl commands and releases it at the end.
If the device has been removed, always returns -ENODEV.

Signed-off-by: Valentine Barshak <vbarshak@mvista.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>