usb: gadget: f_dfu.c: fix memory leak

dfu_prepare_function() allocates N+1 descriptor header structures,
the last one being the "DFU Functional Descriptor".

dfu_unbind() handles de-allocation, but fails to free the final
one (eg. "DFU Functional Descriptor"), leading to memory leak.

Fixed by incrementing counter, as in dfu_prepare_function().

Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>