ALSA: jack: Fix endless loop at unique index detection

While the commit [d0a601c2: ALSA: jack: Fix the id uniqueness
check] fixes the wrong string check, it leads to a worse result -- the
loop in get_available_index() goes into an endless loop.  The cause is
that snd_ctl_find_id() returns the object assigned to the numid if
it's set.  Thus it points to the previous entry again.

This patch clears the numid field for the next call properly.

Reported-and-tested-by: Tomáš Pružina <pruzinat@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>