This patch use devm_extcon_dev_allocate() to simplify the memory control
of extcon device.

Cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>