ALSA: seq: Check the conflicting port at port creation

We didn't check if a port with the given port number has been already
present at creating a new port.  Check it and return -EBUSY properly
if the port number conflicts.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230523075358.9672-22-tiwai@suse.de


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