Bluetooth: L2CAP: Fix using default Flush Timeout for EFS

There are two Flush Timeouts: one is old Flush Timeot Option
which is 2 octets and the second is Flush Timeout inside EFS
which is 4 octets long.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>