Bluetooth: controller: llcp: increase memory for procedures

The default number of buffers (contexts) for locally initiated
procedures was 4. Per default the host initiates 3 procedures
when opening a connection, allowing the application to initiate
only 1 additional procedure in parallel.
This commit increases the default to 6, if auto phy-update and
dle is enabled, increasing the robustness for an application.

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>