Floss: Hcidoc: Remove pending disconnection on reconnection

When host issues disconnection command with reason powering off, the
controller might or might not send a corresponding disconnection event.
In this instance, we process the disconnection when processing the
command rather than the event, and we also store this information in
pending_disconnect_due_to_host_power_off to avoid double report in case
the controller sends the disconnection event.

If the device is later reconnected and disconnected normally, the
variable pending_disconnect_due_to_host_power_off is potentially
already populated from the previous example, and it blocks us from
reporting any disconnection at all.

This CL cleans the pending disconnection variable on device connection.

Bug: 325574179
Test: m -j
Test: Using report 91536846559, verify all disconnection is reported
      for the device with OUI F7:32:6C.
Tag: #floss
Flag: EXEMPT, Floss only changes
Change-Id: I8f647ea4ab1787e125f96bc11d34a06992cfefb3