usb: gadget: f_hid: actually limit the number of instances

There is a predefined maximum number of hid instances, currently 4.
A chrdev region is allocated accordingly, but with configfs the user
can create as many hid function directories as they like. To make
the number of hid instances consistent with the number of allocated minors,
the limit is enforced at directory creation time.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>