net: rfkill: gpio: Fix crash due to dereferencering uninitialized pointer

Since commit 7d5e9737efda ("net: rfkill: gpio: get the name and type from
device property") rfkill_find_type() gets called with the possibly
uninitialized "const char *type_name;" local variable.

On x86 systems when rfkill-gpio binds to a "BCM4752" or "LNV4752"
acpi_device, the rfkill->type is set based on the ACPI acpi_device_id:

rfkill->type = (unsigned)id->driver_data;

and there is no "type" property so device_property_read_string() will fail
and leave type_name uninitialized, leading to a potential crash.

rfkill_find_type() does accept a NULL pointer, fix the potential crash
by initializing type_name to NULL.

Note likely sofar this has not been caught because:

1. Not many x86 machines actually have a "BCM4752"/"LNV4752" acpi_device
2. The stack happened to contain NULL where type_name is stored

Fixes: 7d5e9737efda ("net: rfkill: gpio: get the name and type from device property")
Cc: [e-mail]
Cc: Heikki Krogerus <[e-mail]>
Signed-off-by: Hans de Goede <[e-mail]>
Reviewed-by: Heikki Krogerus <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Johannes Berg <[e-mail]>

net/rfkill/rfkill-gpio.c

···
 static int rfkill_gpio_probe(struct platform_device *pdev)
 {
 	struct rfkill_gpio_data *rfkill;
-	struct gpio_desc *gpio;
+	const char *type_name = NULL;
 	const char *name_property;
 	const char *type_property;
-	const char *type_name;
+	struct gpio_desc *gpio;
 	int ret;
 
 	if (dmi_check_system(rfkill_gpio_deny_table))