extcon: Remove the optional name of extcon device

This patch removes the optional name of extcon device. Instead,
extcon_dev_register() set the device name as 'extcon[number]' naming pattern.
- /sys/class/extcon/[hardcoded device name] -> /sys/class/extcon/extcon[number]

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: MyungJoo Ham <myungjoo.ham@samsung.com>
Cc: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Cc: Graeme Gregory <gg@slimlogic.co.uk>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: Jaewon Kim <jaewon02.kim@samsung.com>