ASoC: mediatek: Convert to generic PCM copy ops

This patch converts the mediatek BT SCO driver code to use the new
unified PCM copy callback.  It's a straightforward conversion from
*_user() to *_iter() variants.  As copy_form/to_iter() updates the
internal offset at each read/write, we can drop the cur_*_idx counter
in the loop, too.

Note that copy_from/to_iter() returns the copied bytes, hence the
error condition is adjusted accordingly.

Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230815190136.8987-20-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>