Fix Long Read during MTU change

When MTU is changed during GATT Read Long procedure, we can receive full
packet using old MTU value, but compare it's length to new MTU. This
would cause us to not issue another read request, and stay with
partially read value.
If the read indeed ended with the length equal to old MTU, we can safely
attempt to read - remote should reply with invalid offset. This means
this fix is harmless, and shouldn't break any existing devices.

Test: none
Bug: 140617318
Change-Id: I29757bef957c94b1abb56d016cded08108047b09