platform/x86: ISST: Simplify isst_misc_reg() and isst_misc_unreg()

platform/x86: ISST: Simplify isst_misc_reg() and isst_misc_unreg()

After commit '1630dc626c87 ("platform/x86: ISST: Add model specific
loading for common module")' isst_misc_reg() and isst_misc_unreg() can be
simplified. Since these functions are only called during module_init()
and module_exit() respectively, there is no contention while calling
misc_register()/misc_deregister or isst_if_cpu_info_init()/
isst_if_cpu_info_exit().

Hence remove mutex and reference counting.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20240731184256.1852840-1-srinivas.pandruvada@linux.intel.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>