ALSA: oxfw: fix regression to handle Stanton SCS.1m/1d

At a commit 6c29230e ("ALSA: oxfw: delayed registration of sound
card"), ALSA oxfw driver fails to handle SCS.1m/1d, due to -EBUSY at a call
of snd_card_register(). The cause is that the driver manages to register
two rawmidi instances with the same device number 0. This is a regression
introduced since kernel 4.7.

This commit fixes the regression, by fixing up device property after
discovering stream formats.

Fixes: 6c29230e

 ("ALSA: oxfw: delayed registration of sound card")
Cc: <stable@vger.kernel.org> # 4.7+
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>