greybus: spi: validate spi master register

Check for error in registering spi master, even though the current code
will fail a little more ahead when trying to register devices in the
master.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reported-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>