usb: gadget: function: uvc: make sure to balance ep enable/disable

If a set_alt() to the same alternate setting that's
already selected is received, functions are required
to reset the interface state, this means we must disable
all endpoints and reenable them again.

This is also documented on our kdoc for struct usb_function

* @set_alt: (REQUIRED) Reconfigures altsettings; function drivers may
*	initialize usb_ep.driver data at this time (when it is used).
*	Note that setting an interface to its current altsetting resets
*	interface state, and that all interfaces have a disabled state.

Signed-off-by: Felipe Balbi <balbi@ti.com>