platform/x86: thinkpad_acpi: Add a comment about 0 in module_param_call()

platform/x86: thinkpad_acpi: Add a comment about 0 in module_param_call()

As per discussion [1] there are only few users of module_param_call() in
kernel which prevent to read module parameters back.

It thinkpad_acpi driver there is even no method do so. Thus, for now,
add just a comment to explain why 0 is used as permissions in
module_param_call().

[1]: https://patchwork.ozlabs.org/patch/713245/

Cc: Richard Weinberger <richard.weinberger@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>