ALSA: seq: oss: Fix error check at system port creation

The system port creation in ALSA OSS sequencer was wrongly checked
against to the port number that can be never negative.  The error code
should be checked rather against the ioctl call.

This patch corrects the error check.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20210617134742.6321-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>