Bluetooth: Limit LE MPS to the MTU value

It doesn't make sense to have an MPS value greater than the configured
MTU value since we will then not be able to fill up the packets to their
full possible size. We need to set the MPS both in flowctl_init()
as well as flowctl_start() since the imtu may change after init() but
start() is only called after we've sent the LE Connection Request PDU
which depends on having a valid MPS value.

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