ACPI: dock: properly initialize local struct dock_station in dock_add()

Commit fe06fba2 (ACPI: dock: add struct dock_station * directly
to platform device data) changed dock_add() to use the
platform_device_register_data() API.

We passed that interface a stack variable, which is kmemdup'ed
and assigned to the device's platform_data pointer.

Unfortunately, whatever random garbage is in the stack variable
gets coped during the kmemdup, and that leads to broken behavior.

Explicitly zero out the structure before passing it to the API.

This fixes the T41 docking button issue:
http://bugzilla.kernel.org/show_bug.cgi?id=15000



Cc: stable@kernel.org
Reported-by: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>