ALSA: firewire-lib: Remove a comment about restriction of asynchronous operation

The comment for fcp_avc_transaction() describes it doesn't support this type
of operation. But it was already supported by this commit.

00a7bb81c20f3e81711e28e0f6c08cee8fd18514
ALSA: firewire-lib: Add support for deferred transaction

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>