TPM: sysfs functions consolidation

According to Dave Hansen's comments on the tpm_show_*, some of these functions
present a pattern when allocating data[] memory space and also when setting its
content. A new function was created so that this pattern could be consolidated.
Also, replaced the data[] command vectors and its indexes by meaningful structures
as pointed out by Matt Helsley too.

Signed-off-by: Rajiv Andrade <srajiv@linux.vnet.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>