secvar: fix endian conversion

unpack_timestamp() calls le32_to_cpu() for endian conversion of
uint16_t "year" value. This patch fixes the code to use le16_to_cpu().

Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>