Fix use of uninitialized value in btm_acl_update_busy_level

Caught by valgrind:

Thread 7 bt_jni_workqueue:
Conditional jump or move depends on uninitialised value(s)
   at 0x5682870: btif_dm_upstreams_evt (btif_dm.c:1800)
   by 0x56817EF: btif_context_switched (btif_core.c:180)
   by 0x56817EF: bt_jni_msg_ready (btif_core.c:290)
   by 0x571D1D7: work_queue_read_cb (thread.c:233)
   by 0x571C1F9: run_reactor (reactor.c:275)
   by 0x571CF3D: run_thread (thread.c:206)
   by 0x4A5B7B3: __pthread_start(void*) (in /system/lib/libc.so)
 Uninitialised value was created by a stack allocation
   at 0x56D9CC6: btm_acl_update_busy_level (btm_acl.c:503)

Bug: 28113318
Change-Id: I21ffa2f6c48d3bb96dcdc71fee6ab74634e57333