Bluetooth: hci: Fix Create Connection Cancel assert

Since the LE (Enhanced) Connection Complete that comes as part of
cancelling a connection does not come through the priority rx thread in
hci_driver.c, the event class was not being calculated and cached.
Hence, calculate and cache the class whenever an event is received as
part of invoking a command.

Fixes #20110.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>