usb: gadget: f_fs: Fix iterations on endpoints.

When zero endpoints are declared for a function, there is no endpoint
to disable, enable or free, so replace do...while loops with while loops.
Change pre-decrement to post-decrement to iterate the same number of times
when there are endpoints to process.

Signed-off-by: Vincent Pelletier <plr.vincent@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>