[ALSA] Fix substream selection in PCM and rawmidi

The PCM and rawmidi substreams can be selected explicitly by opening
control handle and set via *_PREFER_SUBDEVICE ioctl.  But, when
multiple controls are opened, the driver gets confused.
The patch fixes the initialization of prefer_*_subdevice and the
check of multiple controls.  The first set subdevice is picked up
as the valid one.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>