dell-wmi: fix a memory leak

If dell_new_hk_type is true, dell_legacy_wmi_keymap will point to a memory
allocated in setup_new_hk_map().
In this case, the memory is not freed in current implementation.
This patch fixes the leak by kfree(dell_wmi_keymap) if dell_new_hk_type is true.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>