drivers/platform/x86/thinkpad_acpi.c: delete double assignment

Delete successive assignments to the same location.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/

)

// <smpl>
@@
expression i;
@@

*i = ...;
 i = ...;
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Matthew Garrett <mjg@redhat.com>