Bluetooth: Auto tune if input MTU is set to 0

This enables the code to set the input MTU using the underline link
packet types when set to 0, previously this would likely be rejected by
the remote peer since it would be bellow the minimal of 48 for BR/EDR
or 23 for LE, that way it shall be safe to use 0 without causing any
side effects.

This is convenient for the likes of A2DP transport, see:

https://habr.com/en/post/456182/



Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>