misc: fastrpc: Fix device_open when no session is available

This change fixes fastrpc_device_open() when no session is available and
return an error in such case.

Signed-off-by: Thierry Escande <thierry.escande@linaro.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>