tpm: sandbox: Change the return code when device is already open

All the TPM drivers as well as out TCG TIS API for a TPM2.0 device
return -EBUSY if the device has already been opened.  Adjust
the sandbox TPM do return the same error code.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>