Bluetooth: controller: Fix Data Length Update

Before a LL_FEATURE_REQ is sent, verify that the Data Length Update
procedure is still supported as it could have been unmasked as the result
of a previously completed Feature Exchange procedure.

Add unit test to verify the case of enqueuing both a
Feature Exchange procedure and Data Length Update procedure, where the
Feature Exchange procedure would result in the unmasking of the Data
Length Update procedure.

Signed-off-by: Thomas Ebert Hansen <thoh@oticon.com>