serial: bluetooth: Maximize notification payload size based on ATT MTU

By keeping track of run-time ATT MTU of all connected peers, it's
possible to increase the notification payload size; thus increasing
data-throughput sent over Bluetooth.

Note that this assumes the Bluetooth connection is properly configured
(with the corresponding connection parameters) to optimize performance.
This is application-specific and shall be examined on each scenario for
optimal performance.

Signed-off-by: Luis Ubieda <luisf@croxel.com>