extcon: Implement OF-based extcon lookup properly

Platform bus is not the only way to have extcon devices, so current
implementation of of_extcon_get_extcon_dev() is broken. Also using
parent device node only to get device name is quite ugly.

This patch reimplements of_extcon_get_extcon_dev() to do exactly the
same as extcon_get_extcon_dev() but instead of comparing names, compare
node pointers.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
[mszyprow: simplified the code]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>