usb: gadget: f_fs: use complete() instead complete_all()

usb: gadget: f_fs: use complete() instead complete_all()

There is only one waiter for the completion, therefore there
is no need to use complete_all(). Let's make that clear by
using complete() instead of complete_all().

The usage pattern of the completion is:

waiter context                          waker context
  reinit_completion()
  usb_esp_queue()
  wait_for_completion_interruptible()

					ffs_ep0_complete()
					  complete()

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Cc: Felipe Balbi <balbi@kernel.org>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>