software/bios/cmds: fix crc command with L2 cache

Same CRC was always reported if the memory region was in the cache...
Noticed when manually testing spiflash divisor.

litex/soc/software/bios/cmds/cmd_bios.c

@@ -115,6 +115,8 @@ static void crc_handler(int nb_params, char **params)
 		return;
 	}
 
+	flush_cpu_dcache();
+	flush_l2_cache();
 	printf("CRC32: %08x", crc32((unsigned char *)addr, length));
 }