platform/x86: dell-wmi: Rename dell-wmi.c to dell-wmi-base.c

Rename dell-wmi.c to dell-wmi-base.c, so that we can have other
dell-wmi-foo.c files which can be added to dell-wmi.ko as "plugins"
controlled by separate boolean Kconfig options.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>