Fix bluetoothd crash issue

Fix bluetoothd crash issue

Bluetoothd crashes when any application invokes "Introspect"
dbus method call on device-interface hosted by any device object path.

Declaration of device_signals is corrected

call stack
0  0xb6bfc3ee in strlen () from /lib/libc.so.6
1  0xb6bc69e4 in vfprintf () from /lib/libc.so.6
2  0xb6c65db0 in __vasprintf_chk () from /lib/libc.so.6
3  0xb6dbe404 in g_vasprintf () from /lib/libglib-2.0.so.0
4  0xb6da5dac in g_string_append_vprintf () from /lib/libglib-2.0.so.0
5  0xb6da5e7c in g_string_append_printf () from /lib/libglib-2.0.so.0
6  0xb6efbe94 in print_arguments (gstr=gstr@entry=0xb7f521a0,
    args=0xb6f221fc, direction=direction@entry=0x0) at gdbus/object.c:123
7  0xb6efc624 in generate_interface_xml (iface=<optimized out>,
    gstr=0xb7f521a0) at gdbus/object.c:187
8  generate_introspection_xml (data=0xb7f538a0, data=0xb7f538a0,
    path=0xb7f462e0 "/org/bluez/hci0/dev_E4_FA_ED_DF_2C_2D", conn=0xb7f40590)
    at gdbus/object.c:236
9  introspect (connection=0xb7f40590, message=0xb7f44908,
    user_data=0xb7f538a0) at gdbus/object.c:263
10 0xb6efd13a in process_message (connection=connection@entry=0xb7f40590,
    message=message@entry=0xb7f44908,
    iface_user_data=iface_user_data@entry=0xb7f538a0,
    method=0xb6f42324 <introspect_methods>,
    method=0xb6f42324 <introspect_methods>) at gdbus/object.c:282
11 0xb6efd3b8 in generic_message (connection=0xb7f40590, message=0xb7f44908,
    user_data=<optimized out>) at gdbus/object.c:1106

Change-Id: I8bd5c0e575f23f29945df49bbc3a807d55829989