Bluetooth: TBS: Add support for long read with dirty bit

The TBS spec states that if a value is changed during a
long read procedure, then the server shall reject
read requests with offset != 0 with a specific TBS GATT
error until the value has been read from the beginning again
(i.e. with offset = 0).

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>