Bluetooth: Fix incorrect checks for command buffer user data

If the buffer given to hci_cmd_done() is not from the command buffer
then using the cmd(buf) macro is not valid. Simply bail out from
hci_cmd_done() if this is an event that didn't have a matching command
buffer.

Change-Id: Id8357a23a307f4ef3a9214a4e1f7d853a18cb907
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>