extcon: usbc-tusb320: fix kernel-doc warning

Fix the warning:
drivers/extcon/extcon-usbc-tusb320.c:19: warning: expecting prototype
for drivers/extcon/extcon-tusb320.c(). Prototype was for TUSB320_REG8()
instead

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Rong Chen <rong.a.chen@intel.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>