sound: rawmidi: fix opened substreams count

The substream_opened field is to count the number of opened substreams,
not the number of times that any substreams have been opened.

Furthermore, all substreams being opened does not imply that the next
open would fail, due to the possibility of O_APPEND.  With this wrong
check, opening a substream multiple times would succeed only if the
device had more unopened substreams.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>