Bluetooth: DIS: Only read the requested DIS characteristics

The BLE HID code only uses the PNP_ID characteristic from DIS.
Previously the DIS client would read all DIS characteristics
when a read request was made.  This change allows the DIS user to
request certain DIS characteristics.

Bug: 16521059

Change-Id: I09c24718bb2e2da73c518b64c3cbdd07e7d6cf27
Signed-off-by: Adam Hampson <ahampson@google.com>