[linux] Give tap devices a name and bus type

Give tap devices a meaningful name, and avoid segmentation faults when
attempting to retrieve ${net0/bustype} by assigning a new bus type for
tap devices.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/linux/tap.c

 	}
 
 	nic->interface = if_setting->value;
+	snprintf ( device->dev.name, sizeof ( device->dev.name ), "%s",
+		   nic->interface );
+	device->dev.desc.bus_type = BUS_TYPE_TAP;
 	if_setting->applied = 1;
 
 	/* Apply rest of the settings */

src/include/ipxe/device.h

 /** ISA bus type */
 #define BUS_TYPE_ISA 5
 
+/** TAP bus type */
+#define BUS_TYPE_TAP 6
+
 /** A hardware device */
 struct device {
 	/** Name */

src/net/netdev_settings.c

 		[BUS_TYPE_EISA] = "EISA",
 		[BUS_TYPE_MCA] = "MCA",
 		[BUS_TYPE_ISA] = "ISA",
+		[BUS_TYPE_TAP] = "TAP",
 	};
 	struct device_description *desc = &netdev->dev->desc;
 	const char *bustype;
 	assert ( desc->bus_type < ( sizeof ( bustypes ) /
 				    sizeof ( bustypes[0] ) ) );
 	bustype = bustypes[desc->bus_type];
-	assert ( bustypes != NULL );
+	assert ( bustype != NULL );
 	strncpy ( data, bustype, len );
 	return strlen ( bustype );
 }