Bluetooth: Fix configuration of the MPS value

We were accepting values bigger than we can accept. This was leading
ERTM to drop packets because of wrong FCS checks.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Reviewed-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>