extcon: max77693: Make max77693_extcon_cable static

'max77693_extcon_cable' is used only in this file. Hence
make it static.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>