ALSA: pcm: Fix endless loop for XRUN recovery in OSS emulation

The commit 02a5d692 ("ALSA: pcm: Avoid potential races between OSS
ioctls and read/write") split the PCM preparation code to a locked
version, and it added a sanity check of runtime->oss.prepare flag
along with the change.  This leaded to an endless loop when the stream
gets XRUN: namely, snd_pcm_oss_write3() and co call
snd_pcm_oss_prepare() without setting runtime->oss.prepare flag and
the loop continues until the PCM state reaches to another one.

As the function is supposed to execute the preparation
unconditionally, drop the invalid state check there.

The bug was triggered by syzkaller.

Fixes: 02a5d692

 ("ALSA: pcm: Avoid potential races between OSS ioctls and read/write")
Reported-by:  <syzbot+150189c103427d31a053@syzkaller.appspotmail.com>
Reported-by:  <syzbot+7e3f31a52646f939c052@syzkaller.appspotmail.com>
Reported-by:  <syzbot+4f2016cf5185da7759dc@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>