ACPI: dock: Don't eval _STA on every show_docked sysfs read

Some devices trigger a DEVICE_CHECK on every evalutation of _STA. This
can also be seen in commit 8b59560a


(ACPI: dock: avoid check _STA method).  If an undock is processed, the
dock driver sends a uevent and userspace might read the show_docked
property in sysfs. This causes an evaluation of _STA of the particular
device which causes the dock driver to immediately dock again.

In any case, evaluation of _STA (show_docked) does not necessarily mean
that we are docked, so check with the internal device structure.

http://bugzilla.kernel.org/show_bug.cgi?id=12360

Signed-off-by: Holger Macht <hmacht@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>