extcon: Use devm_kcalloc() in extcon_dev_register()

A multiplication for the size determination of a memory allocation
indicated that an array data structure should be processed.
Thus use the corresponding function "devm_kcalloc".

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>