Bluetooth: hci_ecc: Follow BT spec when public key is invalid

Follow Bluetooth specification recommendation of setting the output
of the DH_Key field in the LE Generate DHKey Complete event when the
public key is invalid.

 If the Remote_P-256_Public_Key parameter of the HCI_LE_Generate_DHKey
 command (see Section 7.8.37) was invalid (see [Vol 3] Part H, Section
 2.3.5.6.1), then all octets of the DH_Key event parameter should be
 set to 0xFF.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>