greybus: spi: unregister master on device add fail

When registering devices if any of it fail, just cleanup and release spi
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>