platform/chrome: cros_ec_ishtp: use helper functions for connection

Use helper functions ishtp_cl_establish_connection() and
ishtp_cl_destroy_connection() to establish and destroy connection
respectively. These functions are used during initialization, reset and
deinitialization flows.

No functional changes are expected.

Signed-off-by: Even Xu <even.xu@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Acked-by: Tzung-Bi Shih <tzungbi@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.com>