Revert "Bluetooth: Fix compare logic in ATT read rsp"

This reverts commit 3d08d167.

The Read Response only contains a Characteristic Value that is less than or
equal to (ATT_MTU – 1) octets in length so, condition
(length < bt_att_get_mtu(conn)) is always true, and the full Characteristic
Value will not be read.

Change-Id: I63280e65b4ff5b830295aa9aadebe2758b626912
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>