ASoC: Update cx20442 for TTY API change

receive_buf() was recently changed to return the number of bytes
received but the cx20442 driver wasn't updated to match the new API.
I don't have any hardware but since we don't actually appears to be
listening to the data at all just report that we accepted all the data
that was offered to us.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>