Bluetooth: hci_ecc: Delay event buffer allocation

Allocate the event buffers after the ECDH operations rather than
before them. This way we don't hold on to the buffer for potentially
multiple seconds while the buffer could be used for other things.

Change-Id: I0fcc34ec4bea2265b7df3c1de3587c2a850c974e
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>