sf: sf_ops: Stop leaking memory

It's usually a common pattern to free() the memory that we allocated.
Implement this here to stop leaking memory.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>

drivers/mtd/spi/sf_ops.c

@@ -421,6 +421,7 @@ int spi_flash_cmd_read_ops(struct spi_flash *flash, u32 offset,
 		data += read_len;
 	}
 
+	free(cmd);
 	return ret;
 }