gatt: Use DEFER_SETUP for EATT channels

gatt: Use DEFER_SETUP for EATT channels

This makes use of DEFER_SETUP mechanism to do the following checks
before accepting the connection:

 - Checks a valid device object exits
 - Checks if initiator/central as if the peripheral start connecting it
   may cause collisions.
 - Checks if the limit of allowed connections has been reached.