Bluetooth: 6lowpan: Remove redundant (and incorrect) MPS assignments

The L2CAP core code already sets the local MPS to a sane value. The
remote MPS value otoh comes from the remote side so there's no point
in trying to hard-code it to any value.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>