pcm: Fix forward/rewind support in iec958 plugin

When forwarding or rewinding, the frame counter was not updated. This
could result in corrupted channel status words or misplaced Z-type
preamble.

Signed-off-by: Alexander E. Patrakov <patrakov@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>