iommu/arm-smmu: Make force_stage module param read-only in sysfs

Changing force_stage dynamically isn't supported by the driver and it
also doesn't make a whole lot of sense to change it once the SMMU is up
and running.

This patch makes the sysfs entry for the parameter read-only.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/arm-smmu.c

@@ -246,7 +246,7 @@
 #define FSYNR0_WNR			(1 << 4)
 
 static int force_stage;
-module_param_named(force_stage, force_stage, int, S_IRUGO | S_IWUSR);
+module_param_named(force_stage, force_stage, int, S_IRUGO);
 MODULE_PARM_DESC(force_stage,
 	"Force SMMU mappings to be installed at a particular stage of translation. A value of '1' or '2' forces the corresponding stage. All other values are ignored (i.e. no stage is forced). Note that selecting a specific stage will disable support for nested translation.");