platform/x86: ISST: Check for admin capability for write commands

platform/x86: ISST: Check for admin capability for write commands

In some SST deployments, administrators want to allow reading SST
capabilities for non-root users. This can be achieved by changing file
permissions for "/dev/isst_interface", but they still want to prevent
any changes to the SST configuration by non-root users.

This capability was available before for non-TPMI SST. Extend the same
capability for TPMI SST by adding a check for CAP_SYS_ADMIN for all
write commands.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Link: https://patch.msgid.link/20260107060729.1634420-1-srinivas.pandruvada@linux.intel.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>