extcon: Remove redundant null checking for class

create_extcon_class() is already Null checking.

Signed-off-by: Bumwoo Lee <bw365.lee@samsung.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>