Bluetooth: Fix respecting le_default_mps value

There's a le_default_mps variable that can be modified through debugfs
but it was never actually used for determining our MPS value. This patch
fixes the MPS initialization to use the variable instead of a fixed
value.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>