samples: drivers: spi_flash: fix printf warning on 64bit environment

This patch fixes printf format specifier "%u" -> "%zu" for printing
an argument of size_t on 64bit environment.

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>

samples/drivers/spi_flash/src/main.c

@@ -74,7 +74,7 @@ void main(void)
 
 	printf("\nTest 2: Flash write\n");
 
-	printf("Attempting to write %u bytes\n", len);
+	printf("Attempting to write %zu bytes\n", len);
 	rc = flash_write(flash_dev, FLASH_TEST_REGION_OFFSET, expected, len);
 	if (rc != 0) {
 		printf("Flash write failed! %d\n", rc);