drivers/spi: be sure to call spi_init() before spi_flash_probe()

It's necessary to call spi_init() prior to calling spi_flash_probe()
such that the SPI drivers can do any work required prior to performing
SPI transactions.  It could be argued that the drivers should handle
such situations, however the SPI API implementations seem to assume the
callers ensured spi_init() was called before any SPI transactions.

This fixes systems that failed to boot introduced by [1]. Issue tracked
in https://ticket.coreboot.org/issues/67.

[1] I2aa75f88409309e3f9b9bd79b52d27c0061139c8
    https://review.coreboot.org/16200

BUG=chrome-os-partner:56151

Change-Id: I2d8d5ac685833521f1efe212b07a4b61ba0d9bc3
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/16297
Tested-by: build bot (Jenkins)
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@googlemail.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>

src/drivers/spi/boot_device_rw_nommap.c

@@ -78,6 +78,9 @@ void boot_device_init(void)
 	if (car_get_var(sfg) != NULL)
 		return;
 
+	/* Ensure any necessary setup is performed by the drivers. */
+	spi_init();
+
 	car_set_var(sfg, spi_flash_probe(bus, cs));
 }