Bluetooth: Refactor hci_disconn_complete_evt

hci_disconn_complete_evt() logic is more complicated than what it
should be, making it hard to follow and add new features.

So this patch does some code refactoring by handling the error cases
in the beginning of the function and by moving the main flow into the
first level of function scope. No change is done in the event handling
logic itself.

Besides organizing this messy code, this patch makes easier to add
code for handling LE auto connection (which will be added in a further
patch).

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>