driver/char/tpm: declare internal symbols as static

This patch declares the internal struct and functions as static to provide
more security.

Signed-off-by: Xiaoyan Zhang <xiaoyan.zhang@intel.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Kent Yoder <key@linux.vnet.ibm.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>