extcon: Change field type of 'dev' in extcon_dev structure

The extcon device must always need 'struct device' so this patch change
field type of 'dev' instead of allocating memory for 'struct device' on
extcon_dev_register() function.

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