moli: add USB mux task
Follow brask to add USB_MUX task.
TEST=make -j BOARD=moli
TASK_ALWAYS(USB_CHG_P1, usb_charger_task, 0, TASK_STACK_SIZE) \
TASK_ALWAYS(USB_CHG_P2, usb_charger_task, 0, TASK_STACK_SIZE) \
TASK_NOTEST(CHIPSET, chipset_task, NULL, LARGER_TASK_STACK_SIZE) \
+ TASK_ALWAYS(USB_MUX, usb_mux_task, NULL, VENTI_TASK_STACK_SIZE) \
TASK_ALWAYS(HOSTCMD, host_command_task, NULL, LARGER_TASK_STACK_SIZE) \
TASK_ALWAYS(CONSOLE, console_task, NULL, VENTI_TASK_STACK_SIZE) \
TASK_ALWAYS(POWERBTN, power_button_task, NULL, LARGER_TASK_STACK_SIZE) \
