Bluetooth: controller: verify DLE req_rsp has valid values

During testing, lr->max_rx_octets and lr->max_tx_octets were
at times set to 0.  If we use these 0 values, we end up with
very erratic behavior.  Best, to check for a sane value and
if invalid, default to the value in _radio.conn_curr->max_*x_octets.

Change-Id: I57c0e3790d988f0de17993cebe5c5c2ab0fc07a6
Signed-off-by: Michael Scott <michael.scott@linaro.org>