Bluetooth: Don't response with error to Exchange MTU req if MTU > 517

According to the Core Spec we shall respond to Exchange MTU Request
with MTU parameter set to the maximum MTU that we can receive.
As a Client, we shouldn't send an error if Server's Rx MTU exceeds
517 bytes. Whe should respond with our maximum MTU, because
after negotiation is done, ATT_MTU shall be set to the
minimum of the Client Rx MTU and Server Rx MTU values.

Error will be sent only in case of Rx MTU lower than LE default
ATT_MTU (23).

Change-Id: I9fa4f3fdb9b8129d52fc7b2557129c0894e5d201
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>