Bluetooth: Fix use of deprecated PRIMARY init level

Fix following warning:

  CC      subsys/bluetooth/host/monitor.o
In file included from zephyr/include/drivers/loapic.h:58:0,
                 from zephyr/include/drivers/ioapic.h:22,
                 from zephyr/include/drivers/sysapic.h:20,
                 from zephyr/include/arch/x86/irq_controller.h:33,
                 from zephyr/include/arch/x86/arch.h:28,
                 from zephyr/include/arch/cpu.h:23,
                 from zephyr/include/kernel.h:2458,
                 from zephyr/include/zephyr.h:20,
                 from zephyr/subsys/bluetooth/host/monitor.c:24:
zephyr/subsys/bluetooth/host/monitor.c: In function
    '_deprecation_check_sys_init_bt_monitor_init0':
zephyr/include/device.h:130:16: warning: '_INIT_LEVEL_PRIMARY' is
    deprecated [-Wdeprecated-declarations]
  static struct device_config _CONCAT(__config_, dev_name) __used \
                ^
zephyr/include/device.h:245:2: note: in expansion of macro
    'DEVICE_AND_API_INIT'
  DEVICE_AND_API_INIT(dev_name, drv_name, init_fn, data, cfg_info, \
  ^
zephyr/include/init.h:69:2: note: in expansion of macro 'DEVICE_INIT'
  DEVICE_INIT(_SYS_NAME(init_fn), "", init_fn, NULL, NULL, level, prio)
  ^
zephyr/subsys/bluetooth/host/monitor.c:193:1: note: in expansion of
    macro 'SYS_INIT'
 SYS_INIT(bt_monitor_init, PRIMARY, MONITOR_INIT_PRIORITY);
 ^
zephyr/include/device.h:48:31: note: declared here
 static __deprecated const int _INIT_LEVEL_PRIMARY = 1;

Change-Id: Ie903e3a075f6614b26018be5769be3651f0963be
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>