ALSA: usb-audio: Use unsigned char for iface and altsettings fields

Just for consistency, use unsigned char for iface and altsetting in
allover places.  Also rearrange the field positions of
snd_usb_endpiont and tidy up with some comments.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-35-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>