greybus: es2.c: create dedicated struct for cport_in and cport_out

Instead of keep cport buffers, urbs and endpoints in es1_ap_dev,
move them in two dedicated struct (es1_cport_in and es1_cport_out),
in order to ease the migration to es2 (increase the number of endpoint).

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>