Bluetooth: att: Fix EATT channel security requirements

Core Vol 3, Part G, Section 5.3.2 Channel Requirements states that
"The channel shall be encrypted". It does not mention any additional
security requirements that can be specified bt higher layer profiles.
This enables link encryption requirement for EATT channel.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>