Bluetooth: controller: Implement event masks

HCI event masking allows the host to choose which events are reported to
it to avoid interruption and excessive traffic. This patch implements
masking to drop any non-enabled events as specified by the host.

Jira: ZEP-1769

Change-Id: If09d4aa22b0da8f743fc42a3b0db3f369daaff96
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>