greybus: operation: add helper to retrieve max payload size

Add helper to retrieve the maximum payload size for operations on a
specific connection.

Note that the helper is not inlined due to how the header files are
currently organised, but it is not expected to be called after a
connection has been initialised either.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>