extcon: max8997: Fix NULL pointer exception on missing pdata

Fix NULL pointer exception when platform data is not supplied. The
driver dereferenced pdata pointer where it could be NULL.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: <stable@vger.kernel.org>
Fixes: 810d601f
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>