greybus: audio: Fix incorrect direction value when enabling RX

The direction value passed to gb_audio_apbridgea_register_cport()
in the gbaudio_module_enable_rx() routine is TX and not RX like
it should be so fix it.

Testing Done: Recorded microphone data from a headset.

Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Fixes: c80e7c6fafa5 ("audio: Split helper APIs based on stream direction")
Tested-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Reviewd-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>