mesh: Fix segfault caused by re-enabling of HCI controller

mesh: Fix segfault caused by re-enabling of HCI controller

This fixes the crash that occurs when a controller used by bluetooth-meshd
is removed and then added back again.

Also, correctly restart scanning when the controller is re-enabled.

Backtrace:
0x00005618e754d040 in ?? ()
0x00005618e6e12d9a in io_ready_callback () at mesh/mesh.c:174
0x00005618e6e3d2c8 in l_queue_foreach () at ell/queue.c:441
0x00005618e6e37927 in request_complete () at src/shared/mgmt.c:261

Change-Id: I360e9b1e4c39898d8b3c695e58f03cb89cf8076e
Signed-off-by: Abhay Agarwal <ay.agarwal@samsung.com>