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

There is a predefined maximum number of printer instances, currently 4.
A chrdev region is allocated accordingly, but with configfs the user
can create as many printer function directories as they like. To make the
number of printer  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>