greybus: gb-beagleplay: Remove use of pad bytes

greybus: gb-beagleplay: Remove use of pad bytes

Make gb-beagleplay greybus spec compliant by moving cport information to
transport layer instead of using `header->pad` bytes.

Greybus HDLC frame now has the following payload:
1. le16 cport
2. gb_operation_msg_hdr msg_header
3. u8 *msg_payload

Fixes: ec558bbfea67 ("greybus: Add BeaglePlay Linux Driver")
Signed-off-by: Ayush Singh <ayushdevel1325@gmail.com>
Link: https://lore.kernel.org/r/20231217121133.74703-2-ayushdevel1325@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>