ALSA: usb-audio: Avoid superfluous endpoint setup

After splitting to snd_usb_endpoint_set_params() and *_prepare(), the
skip of each function should be checked with different flags, while we
still use ep->need_setup as the single one.  Introduce
ep->need_prepare for indicating the need of prepare, and also add the
missing check of ep->need_setup at the set_params.

Fixes: 2be79d58

 ("ALSA: usb-audio: Split endpoint setups for hw_params and prepare (take#2)")
Link: https://lore.kernel.org/r/20221009104212.18877-5-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>