Bluetooth: controller: Fix central enc reject

Add support for both LL_REJECT_IND and LL_REJECT_EXT_IND when waiting
for the the response to the LL_ENC_REQ and LL_START_ENC_REQ.

Add unit test to test both LL_REJECT_IND and LL_REJECT_EXT_IND as
responses to LL_ENC_REQ.

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