Bluetooth: Audio: TBS: Fix read buffer

Add common function to handle large GATT reads of strings.
Initialize the buffer pointers in the net_buf which is used for large
GATT reads by bt_tbs_client_read_call_state() and
bt_tbs_client_read_current_calls().
Define the size of the allocated buffer based on the enabled features.

Signed-off-by: Kim Sekkelund <ksek@oticon.com>