spi/acpi: avoid spurious matches during slave enumeration

In the new SPI ACPI slave enumeration code, we use the value of
lookup.max_speed_khz as a flag to decide whether a match occurred.
However, doing so only makes sense if we initialize its value to
zero beforehand, or otherwise, random junk from the stack will
cause spurious matches.

So zero initialize the lookup struct fully, and only set the non-zero
members explicitly.

Fixes: 4c3c595 ("spi/acpi: enumerate all SPI slaves in the namespace")
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: andy.shevchenko@gmail.com
Cc: masahisa.kojima@linaro.org
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Cc: linux-acpi@vger.kernel.org
Cc: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -1953,7 +1953,7 @@ static acpi_status acpi_register_spi_device(struct spi_controller *ctlr,
 {
 	acpi_handle parent_handle = NULL;
 	struct list_head resource_list;
-	struct acpi_spi_lookup lookup;
+	struct acpi_spi_lookup lookup = {};
 	struct spi_device *spi;
 	int ret;
 
@@ -1962,8 +1962,6 @@ static acpi_status acpi_register_spi_device(struct spi_controller *ctlr,
 		return AE_OK;
 
 	lookup.ctlr		= ctlr;
-	lookup.mode		= 0;
-	lookup.bits_per_word	= 0;
 	lookup.irq		= -1;
 
 	INIT_LIST_HEAD(&resource_list);