Bluetooth: hci_ecc: Fix incorrect private_key size

The private_key parameter to generate_keys is supposed to be 32 bytes
in size, i.e. an array of 8 uint32_t variables.

Change-Id: Ia891fd68bfd8662983b898084e5a8524b7a9b8e2
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>