ALSA: line6: Always setup isochronous transfer properties

While not all line6 devices currently support PCM, it causes no
harm to 'have it prepared'.

This also fixes toneport, which only has PCM - in which case
we previously skipped the USB transfer properties detection completely.

Signed-off-by: Andrej Krutak <dev@andree.sk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>