ACPI: dock: remove global 'dock_device_name'

We only use it in one spot, so it probably gets optimized out, but there's
still no need to use a global variable for this.

Signed-off-by: Alex Chiang <achiang@hp.com>
Acked-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>