ALSA: usb-audio: Add missing proc text entry for BESPOKEN type

ALSA: usb-audio: Add missing proc text entry for BESPOKEN type

Recently we've added a new usb_mixer element type, USB_MIXER_BESPOKEN,
but it wasn't added in the table in snd_usb_mixer_dump_cval().  This
is no big problem since each bespoken type should have its own dump
method, but it still isn't disallowed to use the standard one, so we
should cover it as well.  Along with it, define the table with the
explicit array initializer for avoiding other pitfalls.

Fixes: e3eb6a35a9fd ("ALSA: usb-audio: scarlett2: Fix wrong resume call")
Reported-by: Pavel Machek <pavel@denx.de>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210714084836.1977-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>