Bluetooth: Mesh: Fix clearing model subscription list
Bluetooth: Mesh: Fix clearing model subscription list

The code was storing a "cleared" entry in storage for every model,
regardless of them having any subscriptions or not. Update hte
mod_sub_list_clear() function to return a "cleared entries" count so
that the calling code can decide whether any action is needed or not.

Signed-off-by: Johan Hedberg <628991c7b0a19c384f9b99c21ec330ce952bb838@intel.com>

finikorg/zephyr,finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr