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>

drivers/platform/x86/thinkpad_acpi.c

@@ -9664,6 +9664,7 @@ module_param_named(enable, alsa_enable, bool, 0444);
 MODULE_PARM_DESC(enable, "Enable the ALSA interface for the ACPI EC Mixer");
 #endif /* CONFIG_THINKPAD_ACPI_ALSA_SUPPORT */
 
+/* The module parameter can't be read back, that's why 0 is used here */
 #define TPACPI_PARAM(feature) \
 	module_param_call(feature, set_ibm_param, NULL, NULL, 0); \
 	MODULE_PARM_DESC(feature, "Simulates thinkpad-acpi procfs command at module load, see documentation")