Bluetooth: Gatt: Replace callback in gatt subscription callback.

Add a "subscribe" callback in the bt_gatt_subscribe params, and
deprecate the "write" callback.

The purpose of this is to be able to return the subscription
parameters in the callback.  The write callback
- (in principle) returns write parameters
- in fact returns nothing - the pointer is set to NULL

Signed-off-by: Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>