tpm: migrate pubek_show to struct tpm_buf

Migrated pubek_show to struct tpm_buf and cleaned up its implementation.
Previously the output parameter structure was declared but left
completely unused. Now it is used to refer different fields of the
output. We can move it to tpm-sysfs.c as it does not have any use
outside of that file.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>