ALSA: oxfw: fix a condition and return code in start_stream()

The amdtp_stream_wait_callback() doesn't return minus value and
the return code is not for error code.

This commit fixes with a propper condition and an error code.

Fixes: f3699e2c ('ALSA: oxfw: Change the way to start stream')
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Cc: <stable@vger.kernel.org> # 3.19+
Signed-off-by: Takashi Iwai <tiwai@suse.de>