Bluetooth: hci_ecc: Fix ECDH API usage

The main issue is that after the latest update to TinyCrypt the
ecc_make_key() API expects a twice as big random number. Fix this, but
also move the variables to the static context since we're limiting one
HCI command at a time. Also place the variables in a union based on
their temporal dependencies. Thanks to this, we can now further reduce
the ECC stack size by 40 bytes (on ARM this then reports 12 bytes of
unused stack after key-pair generation and DHKey calculation).

Change-Id: I1036e0ca15f7c08063cba9e568d7df99e65c9156
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>