Bluetooth: hci: added h4_close function

Inside the hci driver API in h4.c, there were no
API function allowing to close the bluetooth
interface, as there were close function defined
for other bluetooth hci driver.

The function disables Bluetooth reception and
sending, and calls a weak function allowing
to implement user specific behaviors
while shutting down bluetooth interface

Signed-off-by: FILLIOT Louis <l.filliot@lacroix.group>