tpm/tpm-sysfs: active_show() can be static

so we make it static

CC: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
CC: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>