audio: Fix memory leak when headset server probe() fails

audio: Fix memory leak when headset server probe() fails

If RFCOMM is disabled on the kernel, headset_server_probe() fails.
Relevant log messages:

 audio/manager.c:headset_server_probe() path /org/bluez/499/hci0
 src/adapter.c:btd_adapter_ref() 0x4bb4f78: ref=6
 audio/manager.c:audio_adapter_ref() 0x4ca3010: ref=1
 socket(STREAM, RFCOMM): Protocol not supported (93)
 audio/manager.c:audio_adapter_unref() 0x4ca3010: ref=0
 src/adapter.c:btd_adapter_unref() 0x4bb4f78: ref=5
 audio-headset: Operation not permitted (1)

The powered callback should only be registered if adapter driver probe
was successful. The callback unregister was moved to the beginning of
headset_server_remove() for consistency.

This fixes this memory leak:

==499== 8 bytes in 1 blocks are definitely lost in loss record 44 of 182
==499==    at 0x4826444: malloc (vg_replace_malloc.c:263)
==499==    by 0x4877243: g_malloc (gmem.c:132)
==499==    by 0x488D088: g_slice_alloc (gslice.c:836)
==499==    by 0x488E8A5: g_slist_append (gslist.c:230)
==499==    by 0x18AEEE: btd_adapter_register_powered_callback
(adapter.c:3416)
==499==    by 0x11AF61: headset_server_probe (manager.c:919)
==499==    by 0x18B67B: probe_driver (adapter.c:2033)
==499==    by 0x1908F5: adapter_init (adapter.c:2048)
==499==    by 0x189D20: btd_manager_register_adapter (manager.c:397)
==499==    by 0x1649AF: mgmt_cmd_complete (mgmtops.c:1075)
==499==    by 0x16665E: mgmt_event (mgmtops.c:1780)
==499==    by 0x48B2EFA: g_io_unix_dispatch (giounix.c:162)