ALSA: pcm: oss: Simplify plugin frame size calculations

ALSA: pcm: oss: Simplify plugin frame size calculations

Both snd_pcm_plug_client_size() and snd_pcm_plug_slave_size() do the
almost same calculations of calling src_frames() and dst_frames() in
the chain, but just to the different directions with each other.

This patch simplifies those functions.  Now they return -EINVAL for
the invalid direction, but practically seen, there is no functional
changes at all.

Link: https://lore.kernel.org/r/20200309185855.15693-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>