spi_flash: add missing volatile qualifier for lock flags

http://esp32.com/viewtopic.php?f=14&t=419&p=1901

components/spi_flash/cache_utils.c

@@ -38,8 +38,8 @@ static uint32_t s_flash_op_cache_state[2];
 
 #ifndef CONFIG_FREERTOS_UNICORE
 static SemaphoreHandle_t s_flash_op_mutex;
-static bool s_flash_op_can_start = false;
-static bool s_flash_op_complete = false;
+static volatile bool s_flash_op_can_start = false;
+static volatile bool s_flash_op_complete = false;
 
 void spi_flash_init_lock()
 {