tpm/tpm_crb: cache cmd_size register value.

Instead of expensive register access on retrieving cmd_size
on each send, save the value during initialization in the private
context. The value doesn't change.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Tested-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>