dock: fix dereference after kfree()

dock_remove() calls kfree() on dock_station so we should use
list_for_each_entry_safe() to avoid dereferencing freed memory.

Found by smatch (http://repo.or.cz/w/smatch.git/).  Compile tested.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>