thinkpad_acpi: buffer overflow in fan_get_status()

The acpi_evalf() function modifies four bytes of data but in
fan_get_status() we pass a pointer to u8.  I have modified the
function to use type checking now.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>