Bluetooth: Fix long writes handling

This patch fixes long writes handling. Attribute length error and
invalid offset error for prepare write should be sent in a response
to execute write request.

Change-Id: I9b4cbe0df198df9d685dbd2b328009290ebdc567
Signed-off-by: Grzegorz Kolodziejczyk <grzegorz.kolodziejczyk@tieto.com>