greybus: es2: change (un)map methods to static

Endpoints pair will only be managed by es2 driver.
map_cport_to_ep() and unmap_cport() should be static.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>