platform/x86: dell-wmi: Update dell_wmi_check_descriptor_buffer() to new model

This converts dell_wmi_check_descriptor_buffer() to the new driver
model interface and puts the interface version in dell_wmi_priv
where it belongs.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>