If the requested buffer size of the frontend is smaller than the fixed
buffer size of the host's TPM, fail the startup_tpm() interface function,
which will make the device unusable. We fail it because the backend TPM
could produce larger packets than what the frontend could pass to the OS.

The current combination of TIS frontend and either passthrough or emulator
backend will not lead to this case since the TIS can support any size of
buffer.
Signed-off-by: NStefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>