tpm: fix call order in tpm-chip.c

- tpm_dev_add_device(): cdev_add() must be done before uevent is
  propagated in order to avoid races.
- tpm_chip_register(): tpm_dev_add_device() must be done as the
  last step before exposing device to the user space in order to
  avoid races.

In addition clarified description in tpm_chip_register().

Fixes: 313d21ee ("tpm: device class for tpm")
Fixes: afb5abc2 ("tpm: two-phase chip management functions")

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Reviewed-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>