greybus: spi: fix and cleanup spi devices handling

Cleanup and remove the spi_devices from the greybus spi handling as they
are not needed and they were completely misused.

With this the gb_spi_init does not make sense to exist anymore, so just
remove it and handle the master config directly from connection init.

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>