extcon: cros-ec: Fix a potential NULL pointer dereference

Return -ENOMEM in case of memory allocation failure. This avoids a NULL
pointer dereference.

Fixes: c6983166 ("extcon: cros-ec: Add extcon-cros-ec driver to support display out")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>