Bluetooth: Controller: Fix Tx Buffer allocation for Encryption Request

Encryption request is enqueued in thread context from the Tx
buffer pool, so that it is serialized alongwith the already
enqueued data buffers ensuring they are transmitted out to
peer before encryption is setup. Allocate additional Tx
buffers to accommodate simultaneous encryption setup across
active connections.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>