gatt: Fix EATT MTU

Make sure that EATT MTU is as per Bluetooth Specification

"The ATT_MTU for the Enhanced ATT bearer shall be set to the minimum of the
MTU field values of the two devices; these values come from the L2CAP_-
CREDIT_BASED_CONNECTION_REQ and L2CAP_CREDIT_BASED_-
CONNECTION_RSP signaling packets or the latest L2CAP_CREDIT_-
BASED_RECONFIGURE_REQ packets.
Note: The minimum ATT_MTU for an Enhanced ATT bearer is 64 octets"

Bug: 282719753
Test: Manual
Tag: #feature
Change-Id: I5bb6f7fb36d11c53255de91b927fce226d3c7841