TPM: fix transmit_cmd error logic

It's incorrect to assume that buffers returned by the TPM
10 bytes long are always error reports. This patches
parses the error field in its header instead. The error report
is now being printed using dev_err() instead of dev_dbg(), making
it easier for users to provide more detailed bug reports.

Signed-off-by: Rajiv Andrade <srajiv@linux.vnet.ibm.com>