extcon: Add extcon_get_edev_name() API to get the extcon device name

This patch adds the extcon_get_edev_name() API to get the name of extcon device
because all information inclued in the structure extcon_dev should be accessed
by extcon core API instead of directly accessing the data.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>