eeepc-wmi: fix compiler warning

This fixes the following:

  CC [M]  drivers/platform/x86/eeepc-wmi.o
drivers/platform/x86/eeepc-wmi.c:322: warning: initialization from incompatible pointer type

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Matthew Garrett <mjg@redhat.com>