USB: gadgets: ci13xxx: fix probing of compiled-in gadget drivers

Built-in gadget drivers have NULL-ifed unbind() function. Checking
whether unbind() is NULL will never let any compiled into kernel
driver attach.

Signed-off-by: Artem Leonenko <tikkeri@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>